Adcom 5400 Power Amp Distortion - Need Help

I need some clues to help diagnose the source of some serious distortion in my Adcom 5400 power amp.

When I play CD's the speakers sound as though they are blown. The speakers sound as if the cones of the mid-woofers are torn but that is not the case. The highs are non-existent and the mid-range sounds partly there; grates on the ears. I cannot turn up the volume on the pre-amp past the 1/8th mark...any more than that level and the sound just becomes too distorted for listening. The Distortion Alert signal lights also glow more steadily then. The right Distortion Alert signal light lights up frequently and almost immediately; the left light occasionally. The Thermal Overload signals have not yet reacted.

I have eliminated all other upstream components (Adcom GFP 565 preamp and Sony CD player) as the source of the problem, so I'm sure it's the power amp. I've ordered a schematic and intend to dig into the amp as soon as it gets here. I'd appreciate hearing from someone who can quickly get me close to the source of the problem.