OK, any manufactured product can be bad. Nothing is 100%.
But the problem is more like what sirquack said. It just can not handle the M80's. Since i can find no real info from Onkyo on this amp,plus have no idea what this protection circuit is.
I'd rather go with the QSC, which has a big protection circuit, which is not likely to blow up. Rather then risk a vintage amp.
Basically if the M80's shut down this amp...then the speakers are at fault, and i can send them back. To be honest, i'm not in the mind set to pull all the drivers out of these speakers, to check stuff. At least at this point.

I tried to find any and all info on the Onkyo amp. The only thing is i did find a review, where the guy said, great amp, but don't try to run 4 ohm speakers on it. I now have to believe the guy 100%.