Yamaha 2600 w/ M80s.
I think that the Yamaha RX-V2600 will drive the M80s, it is just a matter of how loud, how long, and how well the receiver is ventelated. Check the Axiom forum for feedback because several members are using a Yamaha RX-V2500 or Denon 3805 to drive the M80s.
Also, the upcomming audioholics review of the Yamaha 2600 should shed some light on this subject.
If you don't have a huge HT room, then the 8 ohm M60's are also a safe bet. Axoim has a good return policy which should allow you to down/upgrade from M80's to M60's.