Typically, in a CDP, tubes are used as cathode followers at the output of the CDP. A cathode follower does not add any gain to the signal voltage and therefore is fairly neutral in its sonic signature. If done well, it can be completely neutral. I think there are or were a few CDPs that did use tubes for gain, such as the Raysonic's. In that latter case it might be worthwhile to use the expensive Siemens CCa's in a CDP.
Here we go with more pure "opinion": I don't like 12AU7s at all. Whenever possible I have replaced them with 6CG7 or 6FQ7, which are identical electrically. Among 12AU7s I have used, Amperex Bugle Boys, real ones, were the best, but still not quite as good as a 6CG7. (You can't just plug a 6CG7 or 6FQ7 into a 12AU7 socket; some re-wiring is required.) I'm talking about gain stages; 12AU7 is probably OK for cathode follower use. It's just my opinion, and I was wrong once back in 1965, maybe. (Kidding.)
Here we go with more pure "opinion": I don't like 12AU7s at all. Whenever possible I have replaced them with 6CG7 or 6FQ7, which are identical electrically. Among 12AU7s I have used, Amperex Bugle Boys, real ones, were the best, but still not quite as good as a 6CG7. (You can't just plug a 6CG7 or 6FQ7 into a 12AU7 socket; some re-wiring is required.) I'm talking about gain stages; 12AU7 is probably OK for cathode follower use. It's just my opinion, and I was wrong once back in 1965, maybe. (Kidding.)