Break-in? Hmmmm....
Don't remember break-in with my M3s, do remember though that I didn't care much for them at first, but that went away for sure within the first day.
When I connected up the M50s I paid attention, and "Break-in" lasted about 20 to 25 minutes. Although they "seemed" to get progressively better(ears?)over the next few days.
I don't know if there's a suggested way to break them in, but after reviewing these posts for a goodly number of years, haven't found anything diffinative from Mgt.(that I can recall)other than break-in(if that's the right term)shouldn't last for more than an hour or so,-don't remember the exact number, but Ian himself mentioned it a couple of years back. (Don't quote me cause I can't quote him)
