Go Preamp. the CD is just a buffer, no gain, little effect from tube rolling it.
Having much experience with my Exemplar/Denon universal player, I would have to disagree with Ed about it being "just a buffer." This particular tube mod uses the 5965/6829/7062 tube. There are substantial differences with which tube is in the circuit, with the Amperex 7062 pinch waist being clearly the best.
I would Elevick as I think he is implying that only gain tubes need be rolled. I find rectifier tubes and other non-signal tubes can greatly affect the sound.
Your's is not the best time for beginning this as most of the old classic tubes are in very short supply now. About two years ago with great effort, I got 15 different pairs of 6SN7 tubes to roll in a Supertex Syrah I had at the time. I doubt if I could do this today.
You can hear some remarkable differences. Good luck.
Tube rolling for the Ah! Njoe Tjoeb? Plug in a Scott Nixon Tube DAC. A friend lent me his recently, and the improvement in soundstage and detail was amazing. I ordered in immediately!
I have applied a significant amount of Dynamat and rope caulk to the chassis and transport of the Njoe Tjoeb, and I think this is contributing to it's performance as a transport. It sounds much better than the Sony DVP-S7000 that everyone raves about as a transport.