By Dave G. 

Different preamp tubes will sound different with different amplifiers. NOS tubes are the best if you can get them. I have half a dozen NOS RCAs that I've been really excited to do something with - they'll probably go into a T-wreck style amp. NOS Telefunkens are amazing, as are Bugle Boys for Fender style amps. Mullards, obviously are great sounding.

There's a lot of new production stuff out there that's very good. I personally like EH better than JJs, but JJs are usually pretty good bang for the buck. Rubys are a little harder to find, but I like those a lot. That being said, the Tung-Sol reissues are probably my favorite right now.

Bottom line, buy a variety of 12AX7s and try them all until you find one you like. Also, try them out in different sockets. Depending upon your amp's topography, there's gonna be a big difference in sound. A plexi will sound totally different with a JJ in V1 and an EH in V2 as opposed to an EH in V1 and a JJ in V2, because in a plexi V1 is used for the parallel normal/bright initial stages and V2 is the CC/cathode follower pair. V3, however, isn't going to matter as much because that's the phase inverter, so I'd go for the lowest noise tube I could find.