Hi Te - welcome. I'll throw in my $0.02 here, and agree with what BigWill and mhorgel have already said.
Axiom has stated that the M60 and M80 are sonically very similar, and that the advantage of the M80 is in increased power handling and higher SPL (plays louder). Some users have reported having greater difficulty in positioning the M80 to their satisfaction. I own M60's in a room similar to yours and have never felt like I am missing anything.
Definitely get a good sub for that room. For great home theatre, you will want one. And given the size of the room, music will be much more satisfying, too.
I had trouble with placement in my house too. What I decided to do was somewhat non-traditional. To get a decent viewing position along the long axis, I put the TV in the window bay on the street-side of the house. Now, that window was only interesting to the dog anyway, because looking at the street activity in suburbia is not exactly high culture. So, we always had the drapes closed anyway. This is not a great photo. Ignore the clutter and the light fixture obscuring the upper right corner.
I'm pleased with the arrangement, so wanted to encourage you to continue to examine that "East Wall" scenario.
