Good question but you are going to get a ton of responces. Although an awful lot of video cables as well as audio will work as a co-ax digital cable the actual spec for a co-ax digital cable is 110 ohms not the 75 ohm viseo cable spec. It's not as simple as all that since the RCA connector used is not a 110 ohm plug either. IMO these factors lead to LOT of debate as to what cable works best for digital. I myself am a big fan of synergy so no matter what you buy make sure you can return it. IMO recommendations on cables are much more valid if they come from someone who has heard them on the same make and model of equipment that you yourself are using. Do you have your Digital source and DAC yet?
To answer your first question last, there are many ways to transfer digital signals but the most common are probably digital co-ax with RCA ends so they look like a conventional audio or video cable, optical digital with a Toslink plug which looks nothing like an RCA plug and IMO is a crappy plug and HDMI cable.
As far as what type of cable to use between a CD player and preamp, assuming you have both options, I would try it both ways as sometimes you will like the DAC inside the preamp and sometimes you may prefer what came inside your CD player.
Hope this helps.
To answer your first question last, there are many ways to transfer digital signals but the most common are probably digital co-ax with RCA ends so they look like a conventional audio or video cable, optical digital with a Toslink plug which looks nothing like an RCA plug and IMO is a crappy plug and HDMI cable.
As far as what type of cable to use between a CD player and preamp, assuming you have both options, I would try it both ways as sometimes you will like the DAC inside the preamp and sometimes you may prefer what came inside your CD player.
Hope this helps.