Following up on JohnK's question and your response, if you raise the M80 centre, won't the tweeters be too high up? I suppose you could always pitch the M80 forward a bit to point the tweeters down.
With regard to standing them up on the 600, the 600 has an assymetrical cabinet. So you have to somehow prop it up to give the M80 a level surface. The other thing that you may want to check with Axiom on if you plan on doing this is magnetic coupling between the 600 and the M80. This may result in stray currents in the voicecoils or even the M80's cross-over.
My advice is to make provisions for elevated mounting but experiment to find the optimum installation.
Regarding distance from the wall, I know they have a couple mounted at the Axiom factory but that's really for background listening for the employees. The general recommendation for rear-ported speakers is a minimum of 2 feet away from the back wall. Now having said that, if you plan on crossing over to the 600 at 80Hz, I don't think that distance is a factor since the ports are really only working at low frequencies.
