Hmm... This thread is tilting me closer to taking a stance that is entirely skeptical of burn-in.
While many replies agreed with the original post, they were agreeing that the HD600 doesn't benefit much from burn-in, not that that burn-in is of little use. Would all you HD600 owners have said the same thing about your burn-in experience if not for the results of this experiment?
In the case of burn-in's true existence, I just can't see one headphone improving 2%, while another improves by magnitudes. Sure, there can be a window, but not one like that. Dynamic drivers are still dynamic drivers.
I'm not here to start a war... I legitimately want to know. A quick, but logical, explanation, and I'll shut up! I promise!
Which is true IMHO is that the overall character of the phones won't change, I mean that burn-in is not magic and slow fat sounding cans won't become speed-light fast ones. But burn-in can get you tighter and more defined bass, smoother treble, a more coherent and focused sound... it depends on the cans, you might even end up disliking the sound of some of them after burn-in. This has already happened to me with some IEMs.