It is not the speakers that are powerful, it is all about what drives them. The M60s sound great as do the M80s and you just have to decide if the sound of the M60s is enough for you or do you want that extra little bit more you can achieve with the M80s.
The biggest difference is do you forsee running the system as a 2 channel and want the fullest range mains as possible, this would be the M80s, if not stick with the M60s, you may lose a little bit of midrange/upper end clarity(?)/feel and lower bass range but really only noticeable in direct A/B testing.
It is a tough decision, I went through it a few years ago and ordered the M60s. They sounded a bit different then my M22Ti's, not as forward, so I sought out an M80 owner in town, Mojo and we did a fairly lengthy comparison between my M22s, the M60s and his M80s.
http://axiomaudio.com/boards/ubbthreads.php?ubb=showflat&Number=207481&fpart=1The Denons run the M80s with no problems and in a space like yours should be all you need. If you get upgradeitis you can always add a 2 chanel amp to drive the M80s at a later date.
