The discerning ones will know which are the better sounding power tubes. Nick did a service to the community but unfortunately drove the price of tubes like the GEC 6as7g sky high and has become almost extinct. However he's not the only one to extol the virtues of such tubes. Stavros in his private correspondence with me, told me the GEC 6as7g is the best you can get out of the 6as7 the world. Having heard them and compare with the other power tubes, I agree and not because everyone is saying so.
Japan produced a lot of good audio gear so I'm not surprised that their tubes are good. There's a legend of Sansui, Yamaha, Luxman, Denon, Pioneer vintage amp lovers who regularly import these amps from Japan. Even speakers. I've heard the Diatone DS-3000 which sounded amazing driven by the Sansui au-alpha 907mr, the amp I eventually bought. Almost bought the Diatone too but my present accommodation is best suited for the Axis VoiceBox S monitor on stands.
Yes, almost extinct. Will I ever find a match for my black base cup getter GEC 6AS7G? I have been holding out hope I will come across one rather than seeking out a pair. I am saying goodbye to my Crackatwoa, which means saying goodbye to my GEC as well, until I can find him a partner for the GOTL.
My top 6AS7 types, in no particular order:
GEC 6AS7G
Western Electric 421A
Tung-Sol 7802
Nippon Electric 6520
Bendix 6080WB