I'm not a big believer in burn in like some people around here. I believe there may be some slight mechanical changes happening, but much/most of that will be inaudible. But I know for sure that we underestimate the complexity of the human brain and auditory system. We treat our ears like they were microphones - calibrated tools with the ability to immediately and consistently record what they hear. And that is technically true on some level. But in reality, the bigger picture is that a huge amount of signal processing is done by the brain once the ears have received the sound. As such, there is a very real process involved with "figuring out" the sound produced by any given headphone. This may be a controversial view in some ways, but I do feel like it accounts for the experiences many of us have, in relation to the measurements which don't really support drastic changes due to burn in.
Having said that, I certainly experienced my own little "burn in moment" with the AS-1b. I had been using it merely with a Sansa Clip+ for simplicity. I thought the bass had good extension but was surprisingly light in impact. So I used the Rockbox EQ to bump it up by 4dB, just to get a feel for how it would take an EQ (turns out it does very well). I was pleased with the resulting kick in the bass.
Later I tried the AS-1b in my big system with the bass and ambient ports at various levels of adjustment. Those allow for some pretty wild bass if you are interested.
Eventually I moved back to the Clip+ and was shocked at the bass extension, volume, impact, etc. It was much improved from my first impressions. I figured I had simply left the bass ports open a tad, but double checked and discovered that was not the case. Then a lightbulb went off in my head - I had left the bass boost active in Rockbox, hadn't I? So I go in and check..... nope, it is not enabled. I had already reset the settings after using the AS-1b the first time around. To my ears/brain, this was a completely different sound. And remember, we are talking about the custom version rather than the ASG-1, so it wouldn't be caused by fit issues either.
This is the type of story that almost has to be experienced to be believed. The only way to experience it (and there is certainly no guarantee) is to put in the hours with that particular headphone or IEM. I never dismiss anything quickly but after this experience I'm going to be extra careful to spend more time and be cautious about my impressions.
Edited by project86 - 3/13/12 at 11:08am