Bob, relax; don't let your frustration cloud your judgment. Any competently designed amplifier, certainly one as fine as the M-504, has a protective circuit which shuts things down when there's a short(or overheating)without damage being done. You shouldn't hesitate to test further to see what the real problem is. Don't be frightened by the 4 ohm monster, but if you want, you can try the M80s at around 8 ohms by playing a 1000Hz test tone, where the M80 impedance is around 8 ohms.

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Enjoy the music, not the equipment.