This is from a forum where part of a book available on Amazon has been quoted. It again shows that once speakers have been used they can take a significant length of time to return to their original state and one of them did not quite make it.
So my new theory of headphone burn in (with added proof) is that headphone's either do indeed burn in (but it is not necessarily permanent) or they do not burn in.
If left long enough between listens, the effects of burn in may have totally disappeared. How long is long? From a few seconds to days. Some headphones may not return to their original state, so can be said to have truly burned in. Some cannot be burned in as they keep quickly returning to their original state. Some are half way houses where, if used regularly may never get a chance to return to their original state. But if used occasionally they do have time to return to their original state.
Which headphone is which, I have no idea.