I agree with Fred, as someone that owned M60's for many years and now upgraded to M80's. I would say the overall sound/signature is very similar, if that makes sense. The M80's have the ability to handle more power and play louder, but that doesn't tell you there is a sound difference. The M60's can play as loud as most people would ever need, they are no slouch.
I guess the subtle differences would be a little better bass section. In my opinion this would have to do with a larger cabinet. In addition, some may find some minor improvements to overall detail, due to the addition of the extra tweeters/drivers.
