why are some receivers rated at 6 OHMs and some at 8 OHMS?...Which is better and do the speakers have to match?
6 OHM verses 8 OHM ...what are the differences?
Read-only archive
The difference is obviously 2 ohms. Why do you ask? 
It's the impedance. You have to match the impedance of the speakers to what the receiver will handle. Since:
Volts = Amps X Ohms
and Volts is a constant, then lower OHMs allow higher wattages. You can see this in amp ratings:
200W into 8 ohms
400W into 4 ohms
800W into 2 ohms
1600W into 1 ohm
8 ohms is pretty standard. You normally have to get into the high end stuff to see anything less than 6. At least that's been my limited experience. Cable size increases as the OHMS decrease.
You don't want to drive 4 or 6 ohm speakers with a receiver that's only rated at 8 ohms - you'll eventually overload it. An overloaded receiver will get very hot.
Hope that helps.
It's the impedance. You have to match the impedance of the speakers to what the receiver will handle. Since:
Volts = Amps X Ohms
and Volts is a constant, then lower OHMs allow higher wattages. You can see this in amp ratings:
200W into 8 ohms
400W into 4 ohms
800W into 2 ohms
1600W into 1 ohm
8 ohms is pretty standard. You normally have to get into the high end stuff to see anything less than 6. At least that's been my limited experience. Cable size increases as the OHMS decrease.
You don't want to drive 4 or 6 ohm speakers with a receiver that's only rated at 8 ohms - you'll eventually overload it. An overloaded receiver will get very hot.
Hope that helps.
And to answer your question, just make sure the receiver's ohm rating is less than or equal to the speaker's ohm rating. Lower is better as long as you're not overloading the receiver or the speaker.
Overall the gist of Allens response is correct.
While the formula is correct, for this application we would use the power formula instead:
watts = current X voltage
the correct statement is:
IF volts can remain constant then the amp will double the power output as you divide the impedance in half.
This can happen if the amp is built correctly but this is a rare breed for sure! Finding a stereo amp that will double in power from 8 to 4 ohms with both channels driven is quite an accomplishment.
The key to low fi and mid fi receivers is using speakers that have a high efficiency rating as measured in DB and is called sensitivity. Around 90DB or better is very nice for these applications. Below 88DB and you will be getting into current or voltage hungry speakers.
How much power you need is directly related to the size of the room, how loud you want it and speaker sensitivity. In general, from about 60 watts on down you are looking at a small room with 225 square foot or less. For larger rooms I suggest something more in the 120 watt range. I guarantee this simplistic formula is bound to vary! For the best response using a dedicated multichannel amp or multiple stereo amps is recommended over a receiver.
To achieve a meaningful difference between one amp and another you need to double the power.
So what brought on this question Jeff? Are you upgrading a system or putting together a new one?
Thanks
While the formula is correct, for this application we would use the power formula instead:
watts = current X voltage
and Volts is a constant, then lower OHMs allow higher wattages. You can see this in amp ratings:
the correct statement is:
IF volts can remain constant then the amp will double the power output as you divide the impedance in half.
200W into 8 ohms
400W into 4 ohms
800W into 2 ohms
1600W into 1 ohm
This can happen if the amp is built correctly but this is a rare breed for sure! Finding a stereo amp that will double in power from 8 to 4 ohms with both channels driven is quite an accomplishment.
The key to low fi and mid fi receivers is using speakers that have a high efficiency rating as measured in DB and is called sensitivity. Around 90DB or better is very nice for these applications. Below 88DB and you will be getting into current or voltage hungry speakers.
How much power you need is directly related to the size of the room, how loud you want it and speaker sensitivity. In general, from about 60 watts on down you are looking at a small room with 225 square foot or less. For larger rooms I suggest something more in the 120 watt range. I guarantee this simplistic formula is bound to vary! For the best response using a dedicated multichannel amp or multiple stereo amps is recommended over a receiver.
To achieve a meaningful difference between one amp and another you need to double the power.
So what brought on this question Jeff? Are you upgrading a system or putting together a new one?
Thanks
Richard, I'm thinking about getting the Pioneer Elite VSX-59txi which is rated at 160 w x 7 at 6 OHMS, unfortunately, my speakers are 8 OHM although they are rated for up to 200 watts. I was just wondering if it would be a bad idea to match up a 6 OHM amp to 8 OHM speakers
This can happen if the amp is built correctly but this is a rare breed for sure! Finding a stereo amp that will double in power from 8 to 4 ohms with both channels driven is quite an accomplishment.
Well, I used this one to verify it:
http://www.krellonline.com/html/m_Class ... 700cx.html
700W into 8 ohms
1400W into 4 ohms
2800W into 2 ohms
Then again, nobody ever accused Krell of being mediocre.
Richard, I'm thinking about getting the Pioneer Elite VSX-59txi which is rated at 160 w x 7 at 6 OHMS, unfortunately, my speakers are 8 OHM although they are rated for up to 200 watts. I was just wondering if it would be a bad idea to match up a 6 OHM amp to 8 OHM speakers
Nope - that's fine. Just don't try it the other way around.
You'll lose some power using 8 ohm speakers so you'll be running around 120 watts per channel, but that's still plenty of power. And as Richard mentioned, the sensitivity of the speakers will also make a big difference.
Richard's formula is definitely more correct - I was just trying to point out the relationship between Ohms and watts and why decreasing one increased the other, so I did the simple version.
Nope - that's fine. Just don't try it the other way around.
You'll lose some power using 8 ohm speakers so you'll be running around 120 watts per channel, but that's still plenty of power. And as Richard mentioned, the sensitivity of the speakers will also make a big difference.
Richard's formula is definitely more correct - I was just trying to point out the relationship between Ohms and watts and why decreasing one increased the other, so I did the simple version.
You'll lose some power using 8 ohm speakers
Only if they are innefficient which is not common for 8 ohmers but there are a few out there.
6-8 is not a huge deal unless the speaker has a low impedance hump which is never reflected in the simplistic impedance rating provided by manufacturers. That kind of info typically only comes with a full review using test equipment; unfortunately rare these days. Stereophile still does these tests and more.
I was just trying to point out the relationship between Ohms and watts and why decreasing one increased the other
They definitely have a proportional relationship but...
Well, I used this one to verify it:
http://www.krellonline.com/html/m_Class ... 700cx.html
700W into 8 ohms
1400W into 4 ohms
2800W into 2 ohms
LOL..........ROTFL......
K......
I thought we were talking about common everyday stuff here, not the Ferrari of audio...
So like how about telling us the price tag on that beast while your at it...
Umm......looks like about $14K MSRP. Or $4K more than my entire home theater including the leather recliners and authentic theater seats!
I assumed that if the receiver was pushing 160 watts into 6 ohms that it would not be able to put out that much power into 8 ohm speakers. This is based simply on the multiple ratings published by the amp/receiver mfrs.
e.g. my Denon 2803 is rated at 125 Watts/channel into 6 ohms but only 90 into 8 ohms. So I was assuming that his would be putting out closer to 120 watts if connected to 8 ohm speakers.
I guess the question really is what's the difference between 125 watts into 6 ohms versus 90 watts into 8 ohms? Is it really more power to the speakers or is it the same power delivered differently?
I assumed that if the receiver was pushing 160 watts into 6 ohms that it would not be able to put out that much power into 8 ohm speakers. This is based simply on the multiple ratings published by the amp/receiver mfrs.
e.g. my Denon 2803 is rated at 125 Watts/channel into 6 ohms but only 90 into 8 ohms. So I was assuming that his would be putting out closer to 120 watts if connected to 8 ohm speakers.
I guess the question really is what's the difference between 125 watts into 6 ohms versus 90 watts into 8 ohms? Is it really more power to the speakers or is it the same power delivered differently?
or is it the same power delivered differently?
Exactly.
In your case lets say your Denon was like that Krell. At 90W into 8 it really should do 180 into 4 and it won
I have 2 audiophile friends and I thank my lucky stars that I'm not afflicted with that disease. It's a never ending search for perfection. At least one of them recognizes that beyond a certain point it's not really "better" - just different.
I think Denon advertises the 6 ohm rating because
a) it looks better
b) it shows they can handle a 6 ohm speaker load
If you buy or make holders to keep the speaker wire from touching the carpet to avoid static electricity - you just might be an audiophile!
I think Denon advertises the 6 ohm rating because
a) it looks better
b) it shows they can handle a 6 ohm speaker load
If you buy or make holders to keep the speaker wire from touching the carpet to avoid static electricity - you just might be an audiophile!
Hey guys, yeah I guess it must be okay to mix speaker ohms. The set I'm considering (Epic-60-Home theater-500) has a 6 ohm center channel speaker and the towers and surrounds are 8 ohms. I went ahead and pulled the trigger on that Pioneer Elite VSX-59txi Amp.
http://www.axiomaudio.com/products.html
http://www.axiomaudio.com/products.html
Hey guys, yeah I guess it must be okay to mix speaker ohms.
If you had the impedance plots it would be very clear that the speaker impedance is constantly changing. Speakers with a flat impedance are extremely rare.
I look forward to your results!
Hey guys, yeah I guess it must be okay to mix speaker ohms. The set I'm considering (Epic-60-Home theater-500) has a 6 ohm center channel speaker and the towers and surrounds are 8 ohms. I went ahead and pulled the trigger on that Pioneer Elite VSX-59txi Amp.
http://www.axiomaudio.com/products.html
There's plenty of factors to consider. THD (total harmonic distortion), "all channels driven" and driven full range, like 20-20hz. Manufacturers play around with these #'s to make their amps look powerfull.
The general rule of thumb is more power= more THD. It's just a question of how much more. That Denon 2803 jumps to 0.9% into 6 omhs (while not adding a whole lot more power) compared to very respectable .05% into 8ohms. If you were dealing with a 2ch music only system, you'd hear a difference. With movies not so much, but one thing to consider is your center channel and how you need clarity for dialogue. You don't want to keep hearing the phrase, "What did he say?" over and over again. The 59txi is rated 160X7 into 6ohm at 0.9%.
You don't ned to spend 14G's on a Krell to get power + low distortion. You only need to keep a close eye on the specs and judge what trade offs you'll accept for your room size, which is probably the biggest factor in judging your power needs. I have a big room with vaulted a ceiling, so I needed power. My Sunfire grand sig. amp puts out 425w all channels driven into 8 ohms, 20-20, 0.5% THD (and a whopping 850w into 4ohms!)
I tested a Denon 3805 and while it had it's merits (feature laden), it seemed to struggle to deliver the goods while the Sunfire was effortless. There's a meter on the front that has never moved, no matter how loud I go. It's a $3000 unit, but I picked it up used for $1700 six years ago and it's still going strong. Of course you'll have to figure in the cost of a pre amp also if you choose to go the seperates route.
http://www.axiomaudio.com/products.html
There's plenty of factors to consider. THD (total harmonic distortion), "all channels driven" and driven full range, like 20-20hz. Manufacturers play around with these #'s to make their amps look powerfull.
The general rule of thumb is more power= more THD. It's just a question of how much more. That Denon 2803 jumps to 0.9% into 6 omhs (while not adding a whole lot more power) compared to very respectable .05% into 8ohms. If you were dealing with a 2ch music only system, you'd hear a difference. With movies not so much, but one thing to consider is your center channel and how you need clarity for dialogue. You don't want to keep hearing the phrase, "What did he say?" over and over again. The 59txi is rated 160X7 into 6ohm at 0.9%.
You don't ned to spend 14G's on a Krell to get power + low distortion. You only need to keep a close eye on the specs and judge what trade offs you'll accept for your room size, which is probably the biggest factor in judging your power needs. I have a big room with vaulted a ceiling, so I needed power. My Sunfire grand sig. amp puts out 425w all channels driven into 8 ohms, 20-20, 0.5% THD (and a whopping 850w into 4ohms!)
I tested a Denon 3805 and while it had it's merits (feature laden), it seemed to struggle to deliver the goods while the Sunfire was effortless. There's a meter on the front that has never moved, no matter how loud I go. It's a $3000 unit, but I picked it up used for $1700 six years ago and it's still going strong. Of course you'll have to figure in the cost of a pre amp also if you choose to go the seperates route.
Guys, I've got my new system from www.axiomaudio.com and my new surround amp from Pioneer and WOW! all I can say is, "MIND BLOWING"....this system ROCKS! The front channel left and right towers are actually 4 ohm and the Pioneer is driving them effortlessly! I am now IN my movies! These Axiom speakers sound so clean! The rear surrounds and center channel are 6 OHM. So I guess the answer to my original question has been answered. Make sure you buy a high quality amp that is rated for at least 100 W X 7 at 6/8 OHM that is heavy with good heat sinks and you can drive just about any speaker. Just make sure the amp is well ventilated.
sweet... 
The Pioneer 59txi is a pretty impressive receiver according to the reviews. It's also the perfect time to buy it as Pioneer is probably gearing up for their next latest and greatest "top of the line" receiver. What are they going to improve on from this model? Probably some bells or whistles that you don't really need. I almost bought the 59txi's little brother, the 55txi for a second system. Now I could have a new 59 for about the same price as the 55 was or a used one for half that. What did you wind up paying for it?
One feature you really should take advantage:
Multi-Channel Acoustic Calibration Circuit (MCACC) identifies and evaluates each speakers frequency response; in time, distance, and volume levels for your specific room's environment and calibrates all channel output creating the most accurate 3-Dimensional multi-channel audio sound experience available anywhere. Advanced MCACC even provides PC connectivity so that graphic representations of your room
One feature you really should take advantage:
Multi-Channel Acoustic Calibration Circuit (MCACC) identifies and evaluates each speakers frequency response; in time, distance, and volume levels for your specific room's environment and calibrates all channel output creating the most accurate 3-Dimensional multi-channel audio sound experience available anywhere. Advanced MCACC even provides PC connectivity so that graphic representations of your room
Tombanjo, I bought the Pioneer receiver at http://www2.bigondigital.com/viewitem.p ... Tag=nextag for 2295.00.... I searched and that was the
cheapest place. If you get the next step down model, get the new 56txi...(it is rated for 110 X 7)...I think it's the replacement for the 55txi. And it can be found online for around $900.00....I think it is even cheaper than the 55txi and it's an upgrade....Yes I am aware of the MCACC feature. (still tweeking the system).... This unit as you probably know also has i-link audio which is pretty cool.....(I have an i-link upscalable dvd player from pioneer). I have my front projector hooked up to my computer so you know I will be using the PC audio feature!!! The speakers from Axiom are incredible!...I am especially impressed with the center channel speaker and Sub....They come in Beechwood, Cherry, Black Oak, and light maple...(I opted for the black)....They offer a 30 day trial because they believe, (and so do I) that you should audition the speakers in your home, not a sound treated booth at a retail store. I won't be sending these babies back!....Their customer service is excellent and shipping was FAST! All I can say is KUDOS to this Canadian manufacturer! These speakers are so good you can smell the singers breath coming though them!... .....Here is the link to their home trial gaurantee...
http://www.axiomaudio.com/guarantee.html
cheapest place. If you get the next step down model, get the new 56txi...(it is rated for 110 X 7)...I think it's the replacement for the 55txi. And it can be found online for around $900.00....I think it is even cheaper than the 55txi and it's an upgrade....Yes I am aware of the MCACC feature. (still tweeking the system).... This unit as you probably know also has i-link audio which is pretty cool.....(I have an i-link upscalable dvd player from pioneer). I have my front projector hooked up to my computer so you know I will be using the PC audio feature!!! The speakers from Axiom are incredible!...I am especially impressed with the center channel speaker and Sub....They come in Beechwood, Cherry, Black Oak, and light maple...(I opted for the black)....They offer a 30 day trial because they believe, (and so do I) that you should audition the speakers in your home, not a sound treated booth at a retail store. I won't be sending these babies back!....Their customer service is excellent and shipping was FAST! All I can say is KUDOS to this Canadian manufacturer! These speakers are so good you can smell the singers breath coming though them!...
.....Here is the link to their home trial gaurantee...http://www.axiomaudio.com/guarantee.html
I found this info about driving 4 OHM speakers with an 8 OHM AMP....Was helpful to me....
http://www.audioholics.com/FAQs/4-ohmsp ... ceiver.php
http://www.audioholics.com/FAQs/4-ohmsp ... ceiver.php
Nearly all receivers have current limiting circuits so whether or not there is an impedance switch is irrelevant. The best way around that is a separate amp with a real power supply that can really handle the load and then those circuits won't be there.
Well, that brings us to the age old battle btw seperates and receivers. Audiophiles claim that you can't cram all those features and switching circuts in the same box with all your power supply without creating noise. They were saying this about 2ch for years, imagine what five or seven does?
With the advent of these new "super duper" big $$$ receivers (some as high as $7000+) come the claims of overcoming this problem. I have never had the opportunity to test out a high end receiver, but I've come to this conclusion. If features like room correction, every DSP mode known to man, and whatever other do dads aren't that important to you, why take the chance on increasing your noise floor? Also inherent in those receivers is the fact that they are more prone to breakdown just by definition. Too much stuff crammed in one box.
For $4000 (retail) on up, and that's the price point where they say they're as good as seperates, you can get a pretty damn good amp and pre amp. The pre/pros these days have just about all of the features you need. A minus is the cost of cabling to connect the two, but then again, access is easier for any reconnecting you want to do back there as opposed to those heavy, giant receivers.
With the advent of these new "super duper" big $$$ receivers (some as high as $7000+) come the claims of overcoming this problem. I have never had the opportunity to test out a high end receiver, but I've come to this conclusion. If features like room correction, every DSP mode known to man, and whatever other do dads aren't that important to you, why take the chance on increasing your noise floor? Also inherent in those receivers is the fact that they are more prone to breakdown just by definition. Too much stuff crammed in one box.
For $4000 (retail) on up, and that's the price point where they say they're as good as seperates, you can get a pretty damn good amp and pre amp. The pre/pros these days have just about all of the features you need. A minus is the cost of cabling to connect the two, but then again, access is easier for any reconnecting you want to do back there as opposed to those heavy, giant receivers.
Hi,
one small query. I am buying the onkyo tx-nr906 europe model. its says 220 w/ch @ 6ohms . the specification is under:
Amplifier
one small query. I am buying the onkyo tx-nr906 europe model. its says 220 w/ch @ 6ohms . the specification is under:
Amplifier
They're fine. Notice the rating says 140 wpc @ 8 ohms. When you use speakers with a lower resistance (6 or 4 or 2 ohms) you get more usable power out of the amplifier (assuming the amplifier is designed to go that low). Looks like you can go to 6 ohm speakers and get a bit more power but 8 is fine. No idea what the different abbreviations are but I'm sure google would find them for you.
I dont know what is FTC, IEC and JEITA difference.
Any time you measure something the method and result is based on a reference - that's all that means. It rarely has any value to consumers but if some one were to try and reproduce those results they NEED to know the reference that was used for testing or the outcome may not match.
I have a Pioneer vsx-919ah Receiver over the years the tolerances in the circuitry has dwindled and the 120 rms is there but you really have to crank it. So I ordered An Onkyo TX-nr709 with 110 Rms The Onkyo supports both 8 OHMS and 6 Ohms.... From what I gather 6 OHMS speakers will push 130 Rms output. My question is should I have all speakers at 6 Ohms? Could I mix it up? Should I just keep the 8 ohm rating. I know your going to say it depends on the technology of the receiver but I do not have this unit yet. The reason I ask is the receiver would have to recognize the impedance from the output to each speaker. How can it possibly do that?. You have different gauges of wires = resistance. On a receiver the amp output is usually a RCA jack with like gauge 6 wire in it to define it. It seems at least to me all speakers should be 6 ohms with lots of Rms wattage support. I mean if your increasing 110 to 130 rms that's a massive increase in power. Sorry,...My question is can I mix the 8 ohm speakers with the 6 ohms speakers rated for the power with out dissolving the tolerance of the board on the Onkyo badly.
Without knowing how many speakers you plan to run, I would just leave everything at 8 ohms and see how you like the sound. If it's 5 or 7 speakers, I think the Onkyo will hold it's own in the power department a little better than the Pioneer. Remember that receivers are rated almost always now at 2 channels driven. 110 watts is still a lot of power. You should easily be able to go deaf using 8 ohms with decent speakers. 
...The reason I ask is the receiver would have to recognize the impedance from the output to each speaker. How can it possibly do that?..
The receiver has an internal DC power supply. The peak power delivered to a speaker
depends on the voltage of that power supply and the impedance of the speaker.
From the Onkyo datasheet, the output power vs. speaker impedance is as follows:
3 Ohms 250 Watts
4 Ohms 220 Watts
6 Ohms 170 Watts
8 Ohms 130 Watts
From this data you can infer that the receiver is using a DC power supply around 30 Volts.
(Power equals Voltage squared divided by Ohms)
It delivers 31.9 Volts when connected into 8 Ohms. Because of internal resistance, the voltage
drops down to 27.4 Volts when connected into 3 Ohms.
The channel connected to 6 Ohms will be 1.2 dB louder than the 8 Ohm channel.
dB = 10 times LOG ( Power6 / Power8 )
http://www.eu.onkyo.com/downloads/1/4/7 ... eet_EN.pdf
Reading this thread made me so glad I checked this a couple of years ago, when I bought the better Yamaha AVR, rated 140 x 7 channels at 8 Ohms, and my speakers are big enough and 8 Ohms to match.
From a practical approach, most amps are designed at a maximum power at some given impedance, be it 8 ohms or 6 ohms or what ever. Maximum power will be transferred into the load (speakers) when the impedance of the amp matches the impedance of the speakers. They don't match you don't get maximum power.
IF you have a 200 watt amp designed to work into an 8 ohm load going to a lower impedance load is not going to get more power out of it in the real world.
Now if and it's a big if, you have a 200 watt amp designed to work into an impedance range of 2 through 8 ohms it's still a 200 watt amp and you just tap off the matching transformer.
You don't normally have an amp that delivers 5 A into an indeterminate load. It'll deliver say 5A into an 8 Ohm load for P=I^2 *R or 25*8 = 200 watts. But that amp is unlikely to be able to deliver more than 5A into a 2 ohm load. Now if you try for a constant voltage where P = E^2/R going to the lower resistance is likely to load down the voltage to well less than what it was at 8 ohms. Besides, you have to have proper impedance matching that will not draw the amplifier section out of the linear portion of it's amplification cycle/curve.
The aforementioned math treats the different loads on an amp as if all remains equal, but it doesn't. A 200 watt amp is a 200 watt amp whether it is *matched* into a 2 ohm load of an 8 ohm load. If you could maintain the voltage across the *LOADs* entire range of impedance then you could get the extra power, BUT it doesn't come for free. You would need a 1600 watt amp to get 1600 watts into that 2 ohm load. You are not going to get it by replacing an 8 phm speaker with a 2 ohm and matching it to the amp.
How about a high quality 6 to 8 ohm transformer if you want a real match although you really don't need one.
IF you have a 200 watt amp designed to work into an 8 ohm load going to a lower impedance load is not going to get more power out of it in the real world.
Now if and it's a big if, you have a 200 watt amp designed to work into an impedance range of 2 through 8 ohms it's still a 200 watt amp and you just tap off the matching transformer.
You don't normally have an amp that delivers 5 A into an indeterminate load. It'll deliver say 5A into an 8 Ohm load for P=I^2 *R or 25*8 = 200 watts. But that amp is unlikely to be able to deliver more than 5A into a 2 ohm load. Now if you try for a constant voltage where P = E^2/R going to the lower resistance is likely to load down the voltage to well less than what it was at 8 ohms. Besides, you have to have proper impedance matching that will not draw the amplifier section out of the linear portion of it's amplification cycle/curve.
The aforementioned math treats the different loads on an amp as if all remains equal, but it doesn't. A 200 watt amp is a 200 watt amp whether it is *matched* into a 2 ohm load of an 8 ohm load. If you could maintain the voltage across the *LOADs* entire range of impedance then you could get the extra power, BUT it doesn't come for free. You would need a 1600 watt amp to get 1600 watts into that 2 ohm load. You are not going to get it by replacing an 8 phm speaker with a 2 ohm and matching it to the amp.
How about a high quality 6 to 8 ohm transformer if you want a real match although you really don't need one.
From a practical approach, most amps are designed at a maximum power at some given impedance, be it 8 ohms or 6 ohms or what ever. Maximum power will be transferred into the load (speakers) when the impedance of the amp matches the impedance of the speakers. They don't match you don't get maximum power.
IF you have a 200 watt amp designed to work into an 8 ohm load going to a lower impedance load is not going to get more power out of it in the real world.
.....
I'm thinking of a push-pull transistor amplifier that is DC-coupled to the speakers.
From the datasheet below, the Onkyo does just that.
From the Onkyo datasheet, the output power vs. speaker impedance is as follows:
3 Ohms 250 Watts
4 Ohms 220 Watts
6 Ohms 170 Watts
8 Ohms 130 Watts
I might be guessing wrong, but I suspect they're not using an output transformer.
I suspect it's a direct-coupled amplifier.
IF you have a 200 watt amp designed to work into an 8 ohm load going to a lower impedance load is not going to get more power out of it in the real world.
.....
I'm thinking of a push-pull transistor amplifier that is DC-coupled to the speakers.
From the datasheet below, the Onkyo does just that.
From the Onkyo datasheet, the output power vs. speaker impedance is as follows:
3 Ohms 250 Watts
4 Ohms 220 Watts
6 Ohms 170 Watts
8 Ohms 130 Watts
I might be guessing wrong, but I suspect they're not using an output transformer.
I suspect it's a direct-coupled amplifier.