Hello bobt,
Sorry you're having problems with your M80s and the Onkyo power amp. Like 2x6spds, I believe the old Onkyo M-series of power amps are very robust and it shouldn't have any problems with a 4-ohm load like the M80s. By the way, there were lots of 4-ohm high-end speakers back in the 1980s, including models from Acoustic Research, Infinity (the Kappa series), B&W, Acoustat, Axiom, Apogee, Duntech, JBL, Linn, Boston Acoustics, Dahlquist, Altec and so on.
Back then, at the AV magazine in Canada that I edited (Sound Canada, then Sound&Vision), we tested a fair number of big power amps, including several Onkyos, some Yamahas (M1000, M800), Perraux, Bryston, Macintosh, and others from SAE,. In fact, in that time, lots of stereo receivers would drive 4-ohm loads with no problems because there were just two internal amplifiers to heat sink and keep cool (not seven!).
Although I never owned an M-series Onkyo power amp, I always admired them and recommended them. I owned several Yamaha power amps with big-ass power output meters, one of which continues to power the M80s on my switching system. It has never shut down, nor has the the mid-level H/K AV receiver I have on the M80s. At the Axiom plant, we have an entry-level ($300) Sherwood Newcastle stereo receiver that drives the M80s to extremely loud levels and has never shut down.
2x6's note about the 504 having protection circuitry that may shut it down with large current flow makes me apprehensive, but still, like others here I think there must be a short or a stray wire in the connections or perhaps a defective driver in the M80s (it's rare, but it happens).
It's only in the last decade that some Onkyo receivers had real problems with ultra-sensitive protection circuitry and the 4-ohm M80. Recent higher-end Onkyos seem to be much more stable.
The M-504 in the mid 1980s was expensive in Canada--$1500--and it's mystifying to me that it's having shut-down problems.
Regards,
Alan
