Any cdp with a standard 2v output will drive your passive preamp just fine. Just a matter of flavor after that.
The tube buffer (I tried one and sold it) simply adds a pleasing kind of coloring and IMO, distortion, to the sonics. If it works for you, though, enjoy
the effect. Probably a hell of a lot better than listening to my Oppo thru a passive alone.
And I should note that there are passives and there are passives. One that I heard (MSRP, $6,000) was much more listenable than the others, as it darn well should be.