I tend to be with the camp that says a "burn-in" or "break-in" period does make a difference. I just had a pair of M3's in the house that had been getting broken in for a month. Initially, I found the speakers to be very bright - to the point where my ears were fatigued very quickly. At the end of the month I did not have such issues as the tweeters seemed to smooth out a little. I would have though that it would have been my ears adjusting, but then I took delivery of a new pair of M60's. Ouch, my ears were hurting again within minutes. I swapped the "broken-in" tweeters from the M3's and placed them in the M60's and I had immediate relief.
I think the woofer(s) loosens up for the most part within a few hours of listening, but (to my ears at least) the tweeters take a bit longer and may even continue to get better over time. I don't think most have ears as sensitive to high freq as mine (damaged them pretty good playing live rock unprotected for 6-7 years) and it seems many aren't bothered at all by the sound out of the box.
I left my radio on to the local jazz/npr station (or occasionally throw a fugazi cd in on repeat) for a few days at moderately low levels when I was at work (didn't want to disturb the wife and little one) and louder when I was in the house.
If they sound good to you out of the box, I wouldn't worry about breaking them in too much. If you do notice some listening fatigue early on, I would try and run them for a bit when you aren't in the room as well.
BTW - the M60's really sound nice. Almost effortless in their production of sound and seems to be plenty of space for all the instruments to come across clean and clear.
Just my 2 cents...
