I still think its most likely psychological burn-in or adjustment. Tyll did some tests on burn-in a while back. There were some measurable changes, but so small that it shouldn't account for the
dramatic differences people hear. Of course, this was just on one headphone, but I believe it was the K701/K702 (which many claim requires hundreds of hours of burn-in...so it's a good candidate to test). On the other hand, I did see a monthly updated on Innerfidelity where Tyll said a headphone had odd distortion measurements that went away after 40 hours of burn-in. I don't think he provided the comparison, though...?
There's also the positive feedback from others claiming burn-in can make large differences, which in turn makes others more likely to "hear" the effects of burn-in.
I think it's almost always psychological adjustment or placebo. At the very least, it's likely a mixture of both (mechanical and psychological break-in). Some headphones might benefit...most others not.
I still haven't heard any benefits from burning in any of the T50RPs I've had.
Anyway, just my opinion on burn-in! Even if we don't agree, it doesn't really affect either of us in the end.