The purpose of a preamp is not to "improve" the sound of the source component. The best a preamp can do is not to degrade the music signal passing through it. Anyway that's how it works with an analog preamp. A fully digital preamp is a whole other story. A digital preamp can definitely be used to "improve" the source's sound. EQ, expansion, time/phase alignment, etc. can all be used to make what goes out sound better than what came in. Assuming well designed software and sufficient DSP power (32 bit or greater), than there is the possibility of no sonic degradation from the insertion of an extra device into the music chain.
My Only Question Is.........
Would someone please tell me the name of the preamplifier(s) on this planet that when inserted into the signal path between any recent Wadia cd player and any amplifier, IMPROVES upon the sound of the forementioned Wadia when compared to running it direct using its digital volume control near its peak. Remeber contestants that the better the preamp the less it imparts to the signal. Keep in mind also that answering this question intelligently would require that you have actually tried this.