Bob, your frustration is understandable, and you've continually repeated that the the M-504 doesn't "like" 4 ohms. It's highly likely that this isn't actually the problem though, since the history of the M-504 indicates that it's a very powerful amplifier, including 4 ohm output.

As others have said, an intermittent short somewhere in the wiring appears to be a more likely problem. This might even be internal wiring in one of the M80s, so as Tom suggested, try driving then one at a time to see if only one of them causes a shutdown.

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Enjoy the music, not the equipment.