My feeling is that M80s as surrounds would be overkill for most processor modes and listening situations. There just isn't enough full-range sound that is sent through those speakers to take advantage of what they're capable of.
The exception would be if you primarily listen to 2-channel music in 7-channel stereo mode (which I do from time to time). In this mode, all speakers get a full-range L or R (or L+R mix) signal pumped through them. In this case, surrounding yourself with M80s would be nothing short of awesome.
For most home theater listening situations though, I would prefer the QS8s because of the enveloping surround they provide. I'm not sure you'd get the same feeling from the 80s alone.
