Danno, for what it's worth, I have been using an Onkyo TX-DS797 (forerunner to the Onkyo 703 - rated at 100 watts/channel, but actually less) to drive 8 ohm mains, a 4 ohm center, and 6 ohm surrounds for several years without any problems whatsoever. This doesn't necessarily contradict what Alan had to say since my room is smaller than yours; my mains, which do most of the work when listening to music, are only 8 ohms; and I don't listen at incredibly LOUD volumes. Loud enough for me, however.
Much as I've been happy with the Onkyo, if I were considering the M80s, I'd probably look for an HK or Denon. If I were running M60s which would certainly work in your room, I'd definitely buy Onkyo again.
I have a set of inexpensive (relatively) separates, and am not completely satisfied with the preamp. I'm considering getting a refurbed Onkyo 703 to use as a preamp with my current amplifier.
If you like Onkyo enough that you really want one, you might consider using a separate 2 channel amplifier, or a pair of monoblock amplifiers to power the M80s. That's a more expensive path, but would allow you to enjoy the considerable features the Onkyo offers. Or, you could go with the M60s which would be a less expensive option.
