So, are you placing the M80s against the 15' wide wall? If so, I think that's the better arrangement for sound quality, although 10' is going to be a bit short for the big towers to resolve. Maybe toe them in a bit.
I followed the conventional wisdom found here and used QS8s as side surrounds and a pair of M2s for back surrounds. Didn't work for me. I'd recommend QS8s all around for both music and HT. Sounds better to me. If you think about it a pair of speakers projecting sound directly at each other (side surrounds) makes little sense, same for back surrounds.
I listen to both DVD Audio and SACD. Before I put the M2s in the back surround position I used a pair of wonderful Mission DS77 surround speakers for back surround duty. They were great. I think 4 QS8s would work best.
