LOL. I have never been quite sure about "burn in" as opposed to maybe your head and ears just adjust to the headphone. There have been noticeable differences over time. The bass seems to reach deeper now than it did when I first got them. Started noticing after about 50 hours or so. But I also had stopped listening to them for a couple of weeks as I was getting adjusted to my ES7's.
I do like most everything about the M50. It requires more volume than a lot of my headphones, but the fit and sound is really good. Not aggressive like my ES7's. I prefer aggressive most of the time, but there are lots of days after work later at night when the fam is asleep that I prefer the less in your face sound.