With so many vintage 6SN7 nos tubes out there, why mess around with adaptors. There are a lot of people that feel that the 6SN7 is one of the best sounding audio tubes ever made for a preamplifier. I’m sure if you spend the money and find a reputable dealer you can trust who tests his tubes and properly matches the tubes that the sound quality will amaze you when compared to the factory tubes. I would recommend the better name brands from the 1950’s or the 1960’s. When you start getting into the middle to later 1970’s the quality starts to drop off. I would focus on the better name brands like Telefunken, Amperex, Siemens, or Mullard. These can be expensive but they can absolutely transform a preamp’s sound quality to a whole new level. Try Brent Jessee’s website. Audiotubes.com. For an extra $10.00 a tube he will properly match them with a curve tracer which is a piece of equipment a lot of these other tube dealers don’t have. I would recommend tubes with the same name brand, made in the same year and with the same date codes. Once you hear a great pair of properly matched, nos vintage tubes, you’ll never go back to using the factory tubes! Good luck.