I found the M60s to be nearly identical in sound and detail when the volume was quite loud(80-90db +). The 60's start to breath and open up alot at the higher volumes. The M80s sound this way at all volume levels. This might give you an indication of what to expect for that future upgrade.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70