On the topic of burn in, sometimes I wonder if it’s something a lot simpler than actual burning in of the drivers - maybe it’s just our ears getting a better/different fit with the shell/tips?
Tips and insertion depth of IEM’s has a pretty drastic effect on sound quality in my experience. And sometimes it’s a very subtle change in how an IEM sits in your ear or how wide a tips bore is that can change the sound.
The other thing I’ve noticed is that my ears stretch and tighten depending on how often I am listening to IEM’s, so if for example I use an IEM for an extended period on one day, the next day I’m able to get a deeper fit in my ear and I notice a difference in sound (more isolated, fuller bass, extra clarity in the treble etc).
Not saying any of this is fact, but this is just something I have noticed and thought maybe some could potentially interpret these subtle changes as burn-in? Thinking out loud here