To be fair, I've only owned two headphones that changed with burn in. The K702 and Pro 900. I'll admit the Pro 900s didn't change that much, and still sounded great out of the box. But the bass definitely toned down, and the treble calmed a little, but never completely unfortunately. I think the Pro 900 burn in is a bit overblown around here, but they definitely do change to a certain degree. The AKG's were truly a night and day change though.
That being said, I never noticed any changes with my DT 880s, AD2000s, and most others. The D2000s changed a tiny bit with their bass boominess, but the biggest change came from the J$ s I put on them. So I believe in both sides of the burn in debate lol-it depends on the headphone!
-Daniel