Ive considered going with possibly a tube DAC, or a tube preamp, but since I dont have the money to spend on both a tube preamp and a new DAC at once, its hard to decide which is more important.
There are a couple of good tube preamps in the $500 range used that would give you a good start. Take a look at the Bottlehead Foreplay kit and the Quicksilver preamp. Audible Illusions has some good preamps as well.
As for DACs, I have heard many good things about the MHDT Paradisea+ from some very reliable people. Worth considering as a $400 tube DAC.