He has an HK AVR-525. In further tests, most of the distortion actually comes from the sub (the 'bottoming' mentioned above describes it well); I forgot what crossover frequency he was using at the time. In 2.0 mode, there is a distortion on the m80s at greater than about 0 dB on this amp's volume scale - but it's not as pronounced without the sub (350). Placement might be a bit of an issue here, as he just got the bits and has yet to sort out exactly where to put the sub.