I have a pair of Sylvania 5751 blackplates but unfortunately one tube is noisy (ground noise independent of volume). Otherwise very nice smoothness and I guess they match well with AT woodies (correction: mistakenly thought that I saw an AT on the picture).
I would like to compare my CS300 with the other models but it is not very probable that I get an opportunity. My guess is that the difference is rather small. The XS has a more efficient power supply and gives a few watts extra output. I believe that could be useful with speakers but not important for headphones. There are also a few capacitors of higher quality in the more expensive model and that should make some difference, but of course the PS maybe not only delivers more power but also sounds better.
I think it is because of the limited power for speakers that the Leben amp also has a bass control, and is intended for small speakers that drop in the low bass. In that aspect the K1000 is very similar to small speakers and I sometimes use the bass control with K1000, only the +3 dB setting and get a little extra punch (or rather compensate for the lack bass extension in the headphone). Not purist, but it works!