Hi darcman,
And welcome. Some years ago, Yamaha AV receivers were not on my list of recommended brands to drive the M80s (the list includes Denon, Rotel, Outlaw Audio, Sherwood Newcastle, NAD).
Recently, however, there have been several of Yamaha's top models that, according to test reports by Audioholics and others, have been stable driving 4-ohm loads and the M80s.
I don't know where your model falls in the Yamaha lineup. Try checking past test reports at Audioholics and SoundandVisionmag.com and see how the Yamahas do when connected to a 4-ohm test load. Some AV receiver brands have overly jittery protection circuitry that will shut them down as soon as they sense increased current flow into impedances below 6 ohms.
Regards,
Alan
