There are only so many ways to describe tube tone and most have become cliches. The Tung-Sol 12AX7 has the gain and drive of a Chinese 12AX7 and the pure tone of a Mullard or Brimar from the U.K.
I’ve had two people come to me recently with amps they thought were in need of complete overhauls. In both cases, careful examination revealed no serious problems and all the tubes “tested” as good. At the end of the day, I replaced the NOS Mullard and RCA pre-amps (one in each amp) with a reissue Tung-Sol 12AX7. In both cases the owners were very impressed and thought that their amps had been restored to full health. Believe it or Not.