I love my M60s and I'm sure I'd love M80s. Axiom regularly states the differences are minimal with a bit of the bass extension and ability to play a bit louder going to the 80s.
Something that others have said before that may or not be extremely noticeable, is that the M80s slighter higher attention to high frequency detail is less forgiving to poorly recorded music or the compression in MP3s. So if you are the type to mostly play from your Ipod or your PC, maybe M60s have a slight advantage there. Never compared myself so I'm just repeating others.
I did however certainly learn to destest MP3s on my M60s so much that I spent months re-ripping all of my CDs to uncompressed flac format so I could continue playing music from the PC with my Squeezebox and not hear the difference. Now I just hate poor music producers who compress it before you even buy it.
It's a conundrum that the better a speaker is at accurately reproducing sound exactly as it was recorded, the bigger the number of CDs in your collection you will start to notice flaws in. The good sound sooooo good though.
