Ahhhhh, upgradeitis, I have listened to both side by side and have found the M80 to sound like the M60 does when it is played quite loud, only the M80 sounds like this at all levels low or high SPL. The M80 has a more open/forward sound the M60 doesn't achieve till it gets to higher SPLs, IMO. There are other more subtle things like bass response and such but for me it was the forward sound at all times I liked.
Something else you have to consider is the M80 is 4 ohm speaker and not all amps like this lower load, so thinking about the M80 might be a moot point if you amp won't drive them.
