After an extensive burn-in, the x3i now sound very good, it sounds much more balanced now, compared to when its new. The only complaint I have now is a bit too much bass.

Burn in some more, and the bass will come back again, although much more controlled and tighter, but still a bit too much.

The burn in process I went through with the bass was:
Too much, uncontrolled flabby bass --> Too little, but tighter --> Alot less than first but still a bit too much, much more controlled and tighter.