My M2s sounded awful when I first got them but after a couple of daysthey really sounded nice. Ditto with the M60s but I didn't have time to monitor "sound vs. time", I just let 'em play during the day while I was at work.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8