Yea... I definitely think there is something to be said about the HD-650's getting broken in. The bass becomes a little more relaxed, but not necessarily loose. It's still tight. The headphones opened up considerably, and the music seems to be more enjoyable/impactful.
That is, unless I'm imagining things. I definitely like the HD-650 more than when I first had it, though.
I would certainly agree with breaking in on the 650. Last week I noticed this. I honestly have no idea how many hours I have on my pair. I've only had them for about 7 months, but it sounds like I hit some number of hours that allowed them to cross the threshold to another level. I never did a "burn in" with them.