He sent the speakers to be tested, and apparently they are fine.
Since both channels appear to have the same problem, this has to be a power supply problem with the AVR. Since it sounds as if the receiver is clipping at too low a power level, this sounds like a low voltage problem on the power bus. This is the only way I can put this together from what we know now.
And yes, I'm not keen on AVRs. As far as I'm concerned most of them no longer being for ohm stable at power is highly limiting. You can make a two way bookshelf keep out of trouble, but it is really limiting in the design of large multi driver systems. An awful lot are actually four ohm and sometimes less, and the spec. is not truly honest by the manufacturer. I say not truly honest, because as you know the impedance of a speaker is all over the map, depending on which frequency you want to pick. So, a manufacturer can claim impedance reading from any point on the frequency spectrum. Any reasonable amplifying device should be able to power any reasonably designed speaker within its power band. At one time that was generally true, and now it is the exception among receivers. That is actually intolerable.