It has been said that break-in, the phenomena of equipment improving its sound quality with use, has more to do with breaking in the user's ears than the kit.
I think that opinion is misleading.
There is a difference in equipment performance with break-in. Of this I am certain. I can almost always tell when a unit is fresh off the line or has been in service for some time. It only takes a single audition.
That's not me breaking in. It is the equipment changing with use.
That said, here's an uncomfortable truth. Our ear/brains break-in over time too. The longer we listen the more accepting we become of sound.
While both are true, one does not negate the other.
It's tempting to dismiss the equipment break-in observation because equipment break-in is hard to explain. Often a mystery.
Mysteries are there to solve, not dismiss.