I don't have any experience with 12xxx tubes, but I own both the EH and Sovtek 6SN7s, and I can tell you that they are definitely not the same tube, either in construction or sound. In my Wright WLA12A the EHs are very superior to the Sovteks, and close enough to NOS tubes I've tried that I don't worry about whether the supply of NOS gets too expensive. The Sovteks sound pretty bad in comparison, grainy, constricted, no bass. If only Sovtek existed and not EH, I'd be hoarding NOS 6SN7s whenever I could.
David
David