Hi VegasVec,
Excellent advice from all (bridgman, JohnK, kcarlile) and I have little to add except:
Do not use in-ceiling or in-wall speakers, unless you want mediocre sound quality that tonally will not match the neutral M60s, QS8s, and center channels.
kcarlile is correct: Using as many drivers as the M80 does (six), it is electrically difficult to keep the impedance curve of the system above 4 ohms.
If you want the M80s and wish to drive them hard, do not use an A/V receiver. Get a separate power amp, which will drive 4-ohm loads. Otherwise, the Denon and H/K receivers are fine with the M80s. With your room size, you'd be just as happy with the M60s. Running with a sub, there is very little difference between the M60s and M80s.
Go to the Axiom Archives and Audio Tips and Tweaks for more detail on impedance issues and related topics.
Regards,
