What is a very nice sounding tube preamp?

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.

The OP has probably found a preamp however I feel that I must offer my 2cents:

I finally found a Vac Standard Preamp. I entered the Vac family in October of 2012 with a Renaissance 30/30mkII and the Standard seemed like a good choice. Speakers are Silverline Sonatina I.

Standards/LEs don't come up for sale often and it's easy to see why. The Standard seems to be known for its soundstage and musical presentation. These were immediately apparent.

Just as soon as I had accepted the Standard into my system. I found a Standard LE which I bought. After 2 months of swapping back and forth I decided to keep the Standard LE because it has a remote.

As noted in almost every VAC thread I've read: Kevin Hayes and his company are a wealth of information and stand behind their products.

I love my Standard (and LE).