@Wildcatsare1-The tubes I substituted are Hytron 6SN7GT (got them from Arizona Tube Supply, NOS), and the 12AT7 is Siemans E81CC 3-MICA Munich, from Tubemonger.com), there are others more versed in tubes, with more to pick from and audition than I have (also tried RCA 6SN7GT, didn't love them, but I didn't love any of them singly, that is, the 6SN7 NOS with the stock 12AT7, or the NOS 12AT7 with the stock 6SN7, things came together when I used both the Hytron and Siemans NOS tubes at the same time. Never tried the RCA's with the Siemans, maybe that combination also works, even though the RCA with the stock 12AT7 didn't for me. That's all I know so far...