Burn in isn't a myth. It's a simple mechanical fact. Whether you need to do it can vary - not all companies do their own burn-in - some (Beyer, I believe) do, and some (Sennheiser) don't. But it still gets done at some point (with speakers as well as cans, FWIW).
There's no mistaking that the HD600s I bought sound *massively* diferent to the cans I have now. They are the same cans - as in, the same pair of headphones - but they do not sound like those I first tried. Switching back to the HD270 after months on the HD600 showed how much things had changed. That is, the first time I tried them they sounded way different. A few months later they sounded much more similar. HD600 imaging = another level though, amongst other things. I gave them about 100 hours or so before listening again. They seemed to settle down after 150-200 (?) hours, but there is no doubt the sound changed in that initial time. I was severely disappointed in that first listen.
IMO/IME - leave them in a drawer and play a wide variety of tunes, but errring towards lots of bass and dynamics. Start them at a normal volume, and over the course of a week, nudge the level up a touch every morning before you go to work or whatever. By the end of the week they will be silly loud. See tip and tricks here;
http://www.meier-audio.homepage.t-online.de/
Then get them on your head at a normal level and get listening.
Since having this pair of cans I have tried them with different DACs, different amps, different cables, and none of them made as big a difference as time. Perhaps I'm just a audiophile buffoon falling for placebo, but if you hear it and it helps you work, then I'm all for it.