I'm enjoying these headphones very much. I don't know what to make of 'burn-in,' but I can also vouch that these improve with more hours (probably up to a certain point)-- the top-end has smoothed-out and it has an overall better balance and increased sense of refinement.
But as others have commented: their sound is somewhat, or even downright weird, and yet at the same time, so right. How to make sense of that? I suspect it's because, bucking the trend it seems, they focus on the midrange-- the heart of music, sonically-- and also, importantly, instrumental texture (as best I understand that concept). The sounds or tones of instruments have a rich, yet delicate, saturation. They sound more natural or lifelike-- and yet at the same time wonky! It is I think this focus on the texture of the sound combined with the midrange emphasis that makes these cans special, sonically speaking (their looks and build are also extraordinary/unusual).
I also find that they're an interesting study in the interplay between transparency and neutrality. In comparison with my (former) reference headphones, the Grado HP1000/HP2 for example, the AT's are clearly not neutral, and yet more transparent. Similarly with my other fave headphone, the Denon D7000. I used to think transparency and neutrality went in tandem, but not necessarily so, I'm learning. The Beyer T1 is the closest I have to the combination. (I know there are others more advanced-- Stax, for example). Other people here can speak better on the technicalities of these things. I'm just trying to offer one discerning listener's non-specialized perspective.
I don't know what to make of the AT's soundstage/imaging though. They always sound to me like I am listening to headphones-- and yet somehow in their unorthodox way they can get away with it, with aplomb. You simply enjoy listening to music through headphones with these things. What a wonder. What a delight