What is a very nice sounding tube preamp?

Why do you want a tube preamp to go with that equipment? I have tried some tube preamps and have to say I am a bit disappointed after reading about all the hype. I've tried some recent (only a few years old) Audio Research and Cary models, which are highly respected among tube-o-philes. After having experimented with these 2 tube models, I am still debating, but I think I may be coming to the conclusion that I prefer the higher level of transparency and better bass control that my old Forte solid state provides....

A variety of thoughts --

You can go cheap and pick up a Little Dot MkIII headphone amp, which also has a set of preamp outputs. Sam Tellig in Stereophile laughed his evil laugh as he challenged people to try this little $200 gem as a preamp. I tried it -- and I'll be darned, it's warm, lively, and full of old-fashioned tube presence and midrange magic. Definitely not the last word in accuracy or transparency, but it's enjoyable and fun. (The Little Dot is not recommended for use with direct-coupled amps due to lack of capacitors blocking potential DC output.) Often sold used on head-fi, and available new on eBay (use the official seller, davidzhezhe).

Years ago I used Conrad-Johnson PV12L or CJ-PV10AL. They still show up used every now and then. Spearit Sound is offering the current evolution of the model, called the Classic. The PV-12 and 10 line stages use only two 12AU7 tubes. They add a nice touch of warmth, and, as I recall, roll off the highs a bit.

I also tried Audio Research LS-7. Quite a contrast to the C-J. The ARC sound (with a factory-supplied set of new Sovtek 6922 tubes) was brightly lit, highly energized, almost in your face. Exciting, but over time a bit fatiguing. Which is NOT to say that this description applies to other ARC preamps, new or old.

Finally, I've been reading a lot about the deHavilland UltraVerve 3. Google for many enthusiastic reviews. I wish I could hear this in my own system. I can't speak from firsthand experience, but I am tempted...

I have a Manley Jumbo Shrimp that has excellent spatial and depth of stage characteristics.

I've really enjoyed my Joule LA 100. It's a keeper! Although I'm currently using an Aesthetix Janus Signature, I'm hanging on to my Joule.

If transparency is what you seek, check out SAS Audio labs preamp.