Listener.
Remember: audiophiles think EVERYTHING "burns in."
The audiophile assumption of burn in is a convenient shibboleth for manufacturers to say "you need to keep our speakers for 200 to 500 hours and THEN you’ll know what they really sound like." Foot-in-the-door psychology and all that.
From what I’ve read driver burn in has been measured, but mostly occurs very quickly, as in not long after you play music through them.I’ve seen other measured burn in for some drivers that take longer, but again it seems the implications for audibility seem very minor.
I've visited fellow audiophiles who just got new speakers and put them through the ringer with my own music. A week or more later they say "I think these things have burned in - the seem to have settled down and seem more mellow." I drop over: nope. Sounds exactly as I remembered them. My inference is my friend's ears have simply adjusted over time to the sound.