The guys from OCWorkbench published a guide on modding the Creative Soundblaster X-Fi so the sound quality comes closer to that of cards like the Auzentech X-Meridian:
The aim of this modification is to bring the sound quality of the X-Fi somewhat closer to that of cards like the Auzentech X-Meridian, meaning you will end up with a card with excellent sound quality and retain good gaming performance. Although listening tests are obviously subjective, I have yet to meet anybody who's performed this mod who has not been bowled over by the quality improvement. If you take the time to do it properly, it is unlikely you will regret it. It goes without saying that this mod is irrelevant to anybody using only the digital output of the card, since the DAC + analogue stage will be in the target device and those on the card go unused.