even within headphones, there are many designs. even just looking at dynamic drivers, think about all the materials and shapes used by manufacturers. it's obvious that different gears are simply different. trying to stick some global concept onto all that is a waste of time.
I'm absolutely sure that the Sennheiser guys have run tests or at the very least simulations on hundreds if not thousands of coils and membranes and have measured the hell out of them over time. if only to check for stability and durability. they have the statistically significant results for their very specific model. I'd believe that kind of information if it was made public, no matter what it would say so long as I could see some measurements. even averages would do great.
but average Joe talking from memory about that anecdote from uncontrolled listening, is that a joke? I don't trust my own experience under those conditions, I'd have to be on drug to consider such impressions to have any sort of factual relevance.
how some can have a misplaced confidence in themselves is not helping. TBH anytime I read somebody saying that he's sure he heard a difference, that he has enough experience not to fall for placebo while sticking to sighted impressions, I instantly distrust anything coming from his mouth. I know that 1/ his testing method is unreliable so he doesn't have the means to back up his claims. and 2/ that he's a very bad judge of what is or isn't real.
now I happen like Brooko to measure a few stuff from time to time, those are my anecdotes, and they don't tell me about any mystifying tendency for massive sound change. sometimes a battery needs a full charge or 2, freshly started gear might measure a little differently from when it's been working for some time and warmed up to a stable temperature. headphone pads have the most massive impact I know of on headphones over time. placement, shape, how well they get to conform with our skull, how over time we end up with the driver closer to the ear and all the changes coming from that. they're all very real and very significant changes. at this point I don't have a doubt that pad and bad memory are the 2 most significant causes of change over time on a headphone. so if I come with my large head, I get more clamping, so I need less hours for the pads to give in to pressure. is that still burn in if the size of my head and the headband setting and maybe glasses, determine how much and how fast the sound will change?
for IEMs, dirty dampers(dust and earwax clogging them over time) is a reality, easy to measure, easy to change the dampers for new ones on a few IEMs and check in real time how much we get back at resonance frequencies and in the trebles. it can be many dBs so obviously it's audible. but now if how much ear wax the user generates is what dictates the amount of change, is it still the IEM burning in? ^_^ maybe that's why I'm not a big burn in believer? my body is pretty lazy at secreting anything, from sweat to saliva to earwax. maybe sweat on the pads accelerate changes in elasticity, maybe more earwax would get me to feel IEMs changing rapidly? I'm only half joking, it's kind of a reasonable hypothesis.
shocks can also be unkind to gears. who knew I could burn in my device by violently dropping it on the ground.
if I spent some time thinking I'm sure I could come up with other less significant but still measurable stuff, like plugs, being clean or not, being forced or not, and cables being mistreated, rolled over with a chair, torn on a door knob... those are changes occuring over time but I also have a hard time imagining that I should call it burn in.
those are the most significant changes I have been able to observe objectively. not the membrane of a flexing and "breaking in", not the capacitors being brand new, not my cables being "cooked". doesn't means something can't happens. but I'd already be impressed if the driver of a headphone "burning in" for 300hours could show a change in signature bigger than someone taking his headphone off, and putting it back on. my small anecdotal experience trying to measure stuff tells me that much.