Originally Posted By: PeterChenoweth
Or perhaps this was some sort of manifestation of problems driving M80's. I left the 876 in 6-ohm mode, per suggestions on AVS, so maybe something was going on there.

A very likely issue indeed.
Onkyos are not known for their robust amplifier sections. Although i have run a light 4 ohm speaker on mine in the past, distortion was quickly evident depending on SPL and this occurred at far lower SPL than with a 8 ohm speaker.
Screeching as you suggest, at the more aggressive passages, is exactly how i would have described my experience in the past in a similar situation.

It is too bad you did not have an external amp for comparison with both AVRs to remove that possibility of being a problem.

"Those who preach the myths of audio are ignorant of truth."