Where to hook up DAC?

I'm not sure I completely understand your question, but I will give it a shot. If you want to play music stored on your computer's hard drive (let's say through iTunes), the easiest solution would be to buy a DAC with a USB input. You then connect the PC to the DAC with a USB cable and the DAC to the preamp with audio interconnects, either RCA or balanced.

Some but not all DACs have multiple, selectable digital inputs (e.g. USB, coaxial, optical). If you buy this type of multiple-input DAC and your CD player has a coaxial digital output, you could connect the CD player to the DAC using a coaxial digital cable. However, there is no guarantee that the CD player set up in this way will sound better than if it were connected directly to your preamp. The reason is that good CD players have good internal DACs. You would not want to bypass that internal DAC unless your external DAC (and digital cable) were really good.

If I didn't answer your question or you are still confused, I would go have a look at head-fi.org, where DAC setups are discussed ad nauseum by headphone junkies who use DACs to connect their computers to their headphone amps. By the way, they seem to really like the MHDT Paradisea USB DAC, which I think costs about $900 new.

To clarify I was curious what inputs to use for the DAC. I don't plan on running my cdp through it. However, I found I can "rename" the audio inputs on my pre-amp, so I use one of the other inputs for it. Thanks for your response.