I have XP7 on order. I wounder how A900 will sound with this amp. I've also been avoiding my temptation to get CD3k. Since, I can't addition both, I would like to know how they compare when mated with this amp (especially from owners of emmelines and the two cans).
A900 will improve quite a bit with the XP-7.. however not quite as dramatic as some other headphones might. One of A900's best characteristics (forgiving of the source driving it) also causes to be less affected by amps, unfortunately. However, it's still a good combo, and you should feel some improvement in terms of detail, separation and imaging, and some coloration of sound that makes it even warmer.
CD3000 is much better, as long as your source isn't "thin", alongside this amp, you should be able to clearly see CD3000 is superior in every single way.