Might try a Xiang Sheng 728a. Even with stock tubes it sounds good. Depending on your amps input sensitivity, you may have to attenuate the 728a's out put. It can be a bit much for some amps. A Harrison 6-12 db unit works great.
Swap out the stock 4 tubes (the 2 rectifiers don't effect the sound) for some new production Tung Sol's and you will have a awesome sounding all tube preamp for around $400 or so, that performs way above it's price point.