My original JPL only had the three 12AX7s, which are the amplification and cathode follower tubes. I recall that some JPLs also had a buffer tube for their CD input, that may be your 12AU7's function. If there are three tubes in a row and one by itself near the CD input, that would be my guess. You can e-mail Jadis in France (look under ecrans jadis) with your question, just also give them a serial number and a few days to respond.
Incidentally, while that tube buffer may have been nice for early CD players, I don't think you need it now if you have a good recent CD player. The best sound out of the JPL's inputs in my experience was through the tape input; I'd use that for the CD, unless you primarily listen to vinyl, then use that for the phono stage and AUX for CD.
Incidentally, while that tube buffer may have been nice for early CD players, I don't think you need it now if you have a good recent CD player. The best sound out of the JPL's inputs in my experience was through the tape input; I'd use that for the CD, unless you primarily listen to vinyl, then use that for the phono stage and AUX for CD.