from memory the mids on the S2 don't seem as present as the 30, but i'm sure my opinion will change once i listen side by side...
Yeap. That is exactly the diff. I think if you compare them side by side, its even more obvious. The HD660S2 vocal will seem to have 3 layers of disc foam This was intentional as far as I know, the HD660s2 was tuned for a super relaxed vocal - even more, relaxed than HD650/Hd660. Also note the bass, HD660s2 sub bass will not go very deep, but satisfying enough. NDH 30 bass is deep - planar deep. similar Arya or He1000v2 but the dynamic version of deep bass. I think they compliment each other.