I haven't heard the M80s, but I don't think that dual 6.5" woofers (as good as they sound) can cover bass like a 10 or 12 inch sub. The surface area just isn't there. I'm in the "always have the sub on" camp, so take it with a grain of salt, but there's things you just can't hear without them.

I am the Doctor, and THIS... is my SPOON!