Looking at your posts and what you are looking for, you may want to involve your wife (if you have one) with this purchase. I think that a lot of us on this board have run into the, "what the heck did you bring home" questions and to tell you the truth the subs usually get the most questions. The M600 that people are telling you about is HUGE and in your room, it will stand out. I have read some about the M600 but if it were me I would lean towards an SVS or an HSU unit. I know that I may catch crap for this one but how many reviews have we seen out there on the M600? It does sound great but because of the footprint, and the lack of reviews I would wait a little bit on that one. My room is close to yours and the M60's do a great job with HT. I have not heard the M80's but before you buy anything you may want to audition them both at someones place.
Good Luck!