Well the best preamplifier made by many accounts is the Conrad-Johnson ART, at $20,000. That is probably the most likely to not harm the signal at all. The other option is that perhaps you could use a switching device to switch between the Wadia straight into the amp and the tuner through its own preamp into the amp. A switching device is a lot simpler than a preamp so it should be easier to make it sonically unobtrusive. I do not know if one is made but a company like McCromack might just custome make on for you. The other thing that occurs to me is that there is a home theatre processor loop on the Adcom GTP-750 preamp (which is incredibly good) that you can kick in that takes the signal in and pipes it straight to the output bypassing the volume control. It is intended for home theatre processors but there is no reason you can't do it with a CD player. In fact, now that I have thought of that, I might hook mine up like that ...
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.