I hardly remember what new HD-650s sound like. I habitually burn in everything for 100 hours. I have small rig built for that purpose (2x FM tuners + a DJ mixer). I find that Koss titanium drivers benefit hugely from burn-in. They're cold and distant out of the box, and quit euphonic and fun after a few days of being fed white noise.
I burned in the DT-880s (which were B-stock) and noticed very little difference. Both before and after, they are amazingly comfortable, very graceful, entirely non-fatiguing, but not very involving cans.
I have made no effort to burn in the TakeTs. I also haven't noticed much change as I've used them. More to the point, so many other parameters have changed (amp, interconnects, etc), eliciting major sonic changes (the Fugs are, after all, Ming the Merciless when it comes to revealing what's downstream) that I wouldn't know where to begin attributing what characteristics are the result of "burn-in."