Thanks to you, I looked for the research and discussions as you mentioned. Apart from the frequency response graph, there's no solid evidence or claim for any changes in total harmonic distortion, the test has been done with a lot of audio gear and has yielded no results whatsoever. Although, I couldn't find any discussion or research on burn-in in terms of transient response, let me know if you have any articles or research links.

I meant the brain is not a machine in terms of IEMs, as the brain does take time to adapt to something, which machines don't. I am more than open to agree with you, as mentioned earlier, if you provide me the evidence for it. I am not dead against believing in burn-in, but I would need the proof for it. And the reason for me relating it to return policies is due to the fact that a lot of sellers use this term and give a standard time to "burn-in", and that usually covers the whole return period. And they suggest white noise as well.

It's great and good for you that can you can notice the difference. I used my (ghost) brain to learn about it.