I've had a few over the years, and they can be almost lifeless (is that possible?) with a poor V1 tube.
The best I've ever used was a Mullard ECC83/CV4004; expensive now, but they last forever and it's worth it if you can find and buy one. The Amperex/Bugle Boy was second, and an RCA 12ax7 was third; all to my ears. After that, different tubes made some difference, but not as much; those high-gain Chinese tubes do well in the other positions, but tone is definately improved across the board with better tubes.
Good power tubes also make a big difference, but not as much at lower volumes. I had an Uberschall that was tubed with Mullard XF2's and was the most awesome-sounding amp... I almost always wanted to plug into that amp, over 100b's, SLO's, etc... it was just so powerfull, juicy, and articulate; no mud whatsoever and more powerful-sounding than about anything. The tubes tired out, and I had it retubed with a matched set of hot JJ e34l's. The thing lost so much character and dynamics that I sold it after that... it was not even close to the same amp.