care to give a complete model name, the only stuff google returns is sovtek tubes and a bunch of guitar junk
The DAC on the X-Fi, depending on which X-Fi you have (also kind of something I'm gonna nudge you to provide), should be more than sufficient depending on how picky you are
what monitors are you using, currently?
as far as putting a pre-amp between the soundcard and the monitors, its iffy to say if it'll improve the sound quality, it can't make things better than the amplifiers in the monitors allow, meaning if you get a preamp with insanely good specs, the monitor amps are going to be the limiting factor, however the preamp may add some coloration or distortion which is percieved as favorable (kind of like how Bose speakers are far from being flat or true to music, but because of their coloration/distortion of the audio signals, many people percieve them to be more musical and enjoyable to listen to)
although quantitatively improving the sound quality is probably not going to happen, if its just going analog to analog, the only possible improvement I could see is over the SB X-Fi's volume control, to the preamp's control, but thats honestly not going to be a huge improvement (since the SB X-Fi's volume control still applies itself)
if you were going with a DAC/Preamp like the Grace m902,
m902 reference headphone amplifier it would be taking the digital feed from the x-fi, and converting it to analog, and acting as a preamp, this would be a higher quality solution, however the m902 is inhibitively expensive for most users (at around $1700, it ain't cheap)