I had the opportunity to do an A/B comparison at the Axiom plant, I would agree with what others have said.
As Alan pointed out, the biggest difference is in the bass. The M80 plays the bass stuff with much more authority: more than the specs would seem to indicate. It would be easy to pick the M80s out in any blind tests.
I also compared them to the M22+ sub (M22 is considered more detailed) and here is what I found. The M80 seemed to open up the top end: give it more transparency. Similarly, a sub seemed to open up the bottom end. Noticably so on stuff that had content in the 20-30Hz range like the keyboard stuff on ELPs debut album.
For about a $300 difference, you get noticably extended bass until you buy a sub plus the more subtle differences in the Mids/high end. For me it was worth it.
