GZ34 is indirectly heated. While they work just fine, Dennis recommends directly heated rectifiers. I'll admit that I struggled to find meaningful differences between the Sophia 274B and the EML 274B. I also will be honest in saying that I think I enjoyed the big bottle rectifiers more than the 5Y3GT but I don't know that this has anything to do with better sound, and perhaps it has everything to do with "BECAUSE OMG THEY LOOK SO PRETTY." The 6SN7/6BX7 tubes are another story entirely. Have rolled a dozen different tubes. Still really like the provided (upgrade) NOS 6BX7, had a lot of fun with some NOS RCA and CBS 6SN7. Thought the Sophia 6SN7 was crap. EH 6SN7 was crap. Worth rolling a bit but I honestly would recommend not going crazy: if you find some signal tubes you're happy with, stop dragon chasing. This amp is for listening.