What is needed to have the best CD/Digital sound?

Tubes are certainly not necessary. Some of the best digital i.e. Reimyo, Electrocompaniet, Oracle, Audiomeca et.al is solid state. The design, parts, and build quality is far more important. After that good isolation and cables will take care of the rest. Good luck.

Might want to consider a computer based source - less jitter. There is a lot of info on this topic on audiogon.

Get a SACD/CD player

I've been wondering about this myself these last few months. I read a review on the Monarchy M24 DAC & Linestage in PFO and one criticism the reviewer had is its built-in volume control.

"...Is there a weak point to the M24? Yes. It's the built-in volume control, which knocks down the quality of the linestage quite a bit....If you use the Linestage with the built-in control, it'll sound noticeably more closed-in, less dynamic, and less open than it really is, with just a bit of grain at the top. With an external AVC or TVC feeding the linestage, the linestage actually sounds better than the AVC or TVC by itself, and the sound quality leaps to the top of the tube-linestage league."

What is an external AVC or TVC by itself? And who makes such components?

IMHO you will not hear the best from your cd player without exceptional ac isolation between the cd player and the rest of your system, and exceptional ac conditioning for the cd player. I found a combination of products and power cords to be best.Even power regenerators are sensitive to ac cord upgrades.