The thing that always gets me is that no matter what someone complains about with a headphone, the answer is "it needs to burn in." Literally no matter what the problem is.
Here's my take on it. I'm not a big believer of burn-in. In fact, I have a bias going in that "burn-in is just all in the head", "insignificant", "not going to pass DBT", etc. However, there are 2 of my headphone that actually changes sound in a noticeable way after some play time.
No 1 is this budget, ~$100 IEM. When I first got them, there was no bass. Semi-literally, there is very little bass going on on the IEM. For context, I hate midbass, the more brighter tilt a headphone sound, the better (Beyerdynamic, etc). Then after some time (I wanted to say <10 hours but definitely more than 5), suddenly came the bass. Overwhelmingly even.
Read the IEM's thread and turned out that in earlier production run, the air vent are blocked in some units, and those units have no bass. My unit is way, way later production run (where the price has even increased by $40 by then), and the company rep said that my IEM is not affected by the air vent problem. However, there it was, no bass -> way, way too much bass.
Another one is some brand headphone. I owned several of that brand's headphone at one point. None of them improved with burn-in, at least not in a noticeable
non-hobbyist/average people would notice kinda way. There was this one pair, their $300 pair where when I first got them there was this
envelopingly warm-like sound from them. I can describe the sound perfectly, it's a hd650-light sound. It's like hd650, but brighter. When I first head them, it feels like the midrange is warm and thick and the sound is at all not bright.
After a while, it's suddenly becomes bright (and thus, normal) and sound just like the rest of that headphone brand models.
While I can find a reasonable explanation of why el cheapo IEM may changed sound after burn-in, I still don't have an informed guess why some brand headphone sounds like hd650-light, then becomes normal after some play time. My baseless guess is that the driver needs to stretch? I still don't know why.
So I don't know if the
burn-in is real or not. I still have a bias that burn-in is more like brain adjustment. But I'm more open to the possibility now than I was back then.