Hi Ferguson,
While there is lots of good advice in this thread, especially re. your room size and how loud you want to listen to the M80s, past Axiom customer experience with Yamahas and Onkyos with the M80s suggests that, apart from a few models, those brands are not good choices for 4-ohm loads.
That's the reason Axiom generally recommends Denon, H/K, Rotel, Outlaw and NAD, because they have not had issues with overheating, current limiting or shut-down with the M80s, so long as they aren't used in enormous rooms at extremely loud levels, in which case you'd want to consider big separate power amplifiers of 200 to 300 watts per channel.
Regards,
