The GT-C is basically the Shuguang 9th Gen. Chinese pre tube, but it's tested to high standards. It's got the tightest, focused gain, making it great for highgain amps, although it's not the fattest sounding pre tube. Still a great tube.
The Sovtek LPS has nice gain, a nice overall EQ, and is sweet like a NOS Mullard or Telefunken. It's like a better Sovtek or EH......that's basically what it is...Sovtek's best pre tube.
The Tungsol has a great crunchy vibe to it, making it killer in Marshalls. It's highgain and fat sounding, although it doesn't sustain quite as good as the LPS and GT-C. Well worth the money! It's definitely clearer sounding than the GT-Mullard, and it looks very well made.
The JJECC83 is a clean sounding tube that doesn't do much for a highgain amp, it's best for a clean amp, but the one I mentioned, the ECC803S is JJ's best pre tube. It's fat, gainy, and actually seems to pump up the volume, when you install one in an amp. It seems to have a lot of output, but it's not as sharp and focused as a GT-C. I have the least amount of experience with the JJ ECC803S, but I've liked in in every amp I've tried it in.
The honest truth is that it's very hard to hear any audible difference between these nice hot tubes. I like all 4, and I've swapped these, and many more in all my amps, and these 4 come out the winners. The only tubes that compare to them in quality are NOS tubes like original Made in Britain Mullards and Telefunkens from Germany. I'm just talking about hot pre tubes that sound good in general. For special applications, like vintage Fenders, I'd probably be inclined to stay with RCA, GE, and American 12AX7's, although any of the 4 I mentioned would suit those as well.