~40 hour mark of burn-in:
Wow. These have really opened up and finally sound like they sounded when I first auditioned them at the store.
Now I remember why I bought them. I'm going to revise my review a little bit, because the balance has shot through the roof and these sound almost exactly like Beyerdynamic DT880's right now! The A900X's are quickly becoming my favourite pair of cans. The best part is that I don't -need- an amp!