I compared mine with a new one directly today. There ARE subtle differences, but it could be the pads. The new pads are slightly more full.
The result is the NEW one has a slightly less rounded sound. My old one is more mellow.
I'm not over fussed about the question of burn in (although I have heard things with some headphones, not all) but it could be down to pad thickness perhaps?
The new one seems to lack bass and is edgy. If I turn up, the bass appears, but the sound is too edgy but the old one goes up more comfortably so you feel that there is a better bass.
So, I'll whack the new one with sound for a day or two in order to see whether the treble calms (allowing me to develop more bass) or whether the bass actually forms better after a while......
It's not a massive difference and I'm not able to exactly pinpoint what's different, but there is a difference.