do you think the sound of my system will approach what I would obtain with the M80's and no sub?

I think the real decision lies with whether you plan to listen more to music or home theater... There is a big difference with a subwoofer when watching movies, however I believe the M80s would do well for you (or even M60s) if you are mostly looking for better music reproduction.

----- Clint DeBoer http://www.audioholics.com