Mt10425 -- I don't have any experience with the sound of the 6p1t tube and can't comment on that but just a caution that it's not interchangeable with the EL-84 tube.
This from the tnt-audio review of the Sophia Baby, "The quad of output tubes are Russian military 6P1T pentodes, treated by Sophia for low noise. While the 6P1T is similar in dimensions and outward appearance to the EL84 family of tubes, they are not interchangeable due to significant electrical differences."
This from the tnt-audio review of the Sophia Baby, "The quad of output tubes are Russian military 6P1T pentodes, treated by Sophia for low noise. While the 6P1T is similar in dimensions and outward appearance to the EL84 family of tubes, they are not interchangeable due to significant electrical differences."