I can just tell you my experience with the A21, that has 2 channels instead of three otherwise identical to the A31, but obviously the A21 has a larger power supply on a per channel basis.
It is very well built, though but I could hear the 1200 VA toroidal transformer humming especially if I have any variable speed drive small appliances running such as a humidifier, so I would imagine things like light dimmers might have done the same too. If I run the humidifier at the maximum speed the hum would go much quieter, almost disappeared, but at low speed it was unbearable. Non of my other preamp, amps has audible transformer hum so that's a big negative for me.
In terms of sound quality, we compared it with a little $89 Fosi V3 amp and found no difference, same when compared with the Denon AVR-X3400H, a Hypex NC502MP, a Purifi amp, a Harman Kardon 3390 receiver, and my Bryston 4B SST, they all sound the same. Compared those using REW and Umik-1 mic, the mic heard the same too between 15-22,000 Hz. Anyone who claimed they heard differences between such amps that I owned, compared, I won't argue with them, but I know what facts are and understand how Placebo, expectation bias work. There is just no way any humans can hear things the mic and REW could hear period.
So, based on my own experience, You probably are anticipating that I would suggest you take the simple and stupid approach, that is, grab one of those $89 amp to drive the surround/height speakers, let the AVR drive the front LCR. Or for piece of mind, grab a 200/300 8/4 ohms amp from Audiohphonic, Nord kind of class D amps for the LCR and let the AVR power the remaining speakers, and then just enjoy the music. The A31 is huge and heavy, nicely build but afaic, such things are going to be for the collectors, not for practical use. If I am not downsizing, I would have kept my 4B SST and A21, but at this stage, it was an easy decision to replace them all with the mini amps.
Below is what my mic/REW heard between 3 "separate" amps, one being the Halo A21 and 1 Denon AVR, there can be no scientific (except if Placeo is science too) explanation if anyone could hear that Parasound Halo, class A (up to about 8 W) sound but there must be tons of posts about those different sound signature easily heard between those amps I compared.
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I included some info on class D amps, if you are interested:
Class D power amplifiers | Audioholics Home Theater Forums
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