Sorry, I've been on somewhat of a hiatas from some frustrating audio testing and then thanksgiving and then getting sick.
Generally speaking, I would not pay too much attention to how many amps output. The specifications that say "40A output" are pretty meaningless. The "output watts" is a specification that will indicate the overall size of the power supply. A 500 watt monoblock amp is going to have a rather large transformer and a large power supply capacitor bank. Obviously, you are not going to use all 500 watts for a single speaker, but the benefit is going to be much smoother sound, fuller midrange, and much stronger and fuller bass. It also depends on how hard your speakers are to drive. A smaller 150 watt amp would probably be fine on a more efficient high impedance speaker - such as a flat 8 ohm speaker that is 91db efficient. Once you start going down in impedance and efficiency, you really want a beefier power supply - which translates to a larger power amp.
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Another part is the sonic signature - do you want a very fast responding "Class B" type of amp, or do you want a very warm sounding "Class A" type - or something "neutral" in the middle. It all comes down to personal taste. I just finished my own evaluation of different amps and have heard a whole lot of different electronics when I went to RMAF. In my testing, I would say that the Parasound was the best in fullness and natural sound, but it was too warm for my tastes. Bryston cubed amps were very nice and very high resolution, but were too thin/fast for me (they are just about the closest to a Class B amp you can get). The Classe Audio monoblocks I tested sounded very clinical and sterile in my system, but they did have good resolution.
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So, in my frustration, I just went back to Emotiva and purchased a pair of their 2-channel fully differential amps (the XPA-DR2 models). I have one amp powering the front left / surround left. And the second powering the front right / surround right. Obviously, I'm balancing the power supplies in the amps so that they are used most efficiently, but at a rating of 550 watts per channel, it is totally fine. I will say that after hearing gobs of ultra expensive electronics and a variety of different sonic signature, the new Emotiva differential amps are PRETTY DAMN GOOD! I originally had Emotiva XPR-1 amps (1000 watt monoblocks) for my left/right which I sold earlier this year and regret that. The new amps are only 4 days old and still burning in so I won't have a final judgment for another week or so. It's very difficult to tell if these are going to be better than my original XPR-1 amps or not. I will say my system sounds the best with Emotiva monoblocks (whether they are the old XPR-1 or the new XPA-DR2). The Emotiva are very neutral amps, but they have excellent resolution. The sounds/vocals are very realistic and they are extremely engaging with my B&W D3 speakers (which none of the other amps really gave me). At this point, the amps do not quite have the amount of bass/midbass that I would like, but they are still burning in. They are a slightly different amp board and the power supply is the switching power supply instead of a massive transformer and capacitor bank. That may translate into how much bass strength I have. I will know more in about 4-5 days.
