Personally I'd go for a quality subwoofer, Axiom surrounds, and multichannel audio (SACD or DVD-A) playback before I'd opt for M80s.

However that depends on your listening intentions -- how much home theater vs music, how much legacy stereo vs multichannel music, and your average playback volume.

I could see how a stereo music oriented individual would prefer M80s over M60s, esp if no subwoofer was involved. OTOH if you're more HT and surround music oriented, I think M60s are plenty. Personally in my 20 x 16 room I can't imagine needing more than M60/VP150/QS8 and a sub.

But if you've got the money, get them all -- M80s, surrounds plus a good sub.