Hi bdpf,
And welcome to the Axiom forums.
In addition to the excellent suggestions already made, I'd point out that in the double-blind listening comparisons I do at the Axiom plant, the M80s have audibly deeper bass and more overall bass output than the M60s. They are also ever so slightly smoother on some vocals (mainly operatic solo and massed choral recordings) than the M60s but I'd point out that I'm very fussy and critical about midrange stuff. You would be unlikely to notice the subtle difference on some vocals unless it was an instantaneous comparison between the 60s and 80s.
And the M80s are not too large for the room size you mention. Our Axiom listening room isn't much larger. Since you won't have a subwoofer for a time, I expect you'll really love the powerful deep bass from the M80s on their own. And you can tell the difference between the 60s and 80s with a subwoofer because of the slight midrange differences.
Your Denon 890 will drive either the 4-ohm M80s or the M60s with no stability problems.
Regards,
Alan
