I'd agree with Jack, but I think we've just about done that do death, so I'll agree with Craig instead.
I'm glad your ceiling is not 17' high. That notion - combined with the opening at the back - might have made me waffle just a little about recommending the M60's. Also, like Peter said, cube-shaped rooms are hellish on LF response.
Look, if cost is not an issue, just go ahead and get the M80's. If you (like most of us) have a budget, you'll be completely thrilled with the M60's and a proportionally larger subwoofer investment. I have the 60's and a relatively modest sub (PB12-ISD) in a very large room (relative to cubic feet) and have never been dissatisfied.
You may also want to remember that both models are quite deep, so placement becomes more interesting than it may first appear.
