Never heard of STA and Billington is a tube seller in the UK, so the tubes are probably made for them, but not by them. If you are going to begin tube replacement or rolling, you have more choices with the 6SN7 tubes. You can move toward the venerable VT231, which is a better sounding tube, but more costly. Some people prefer the more laid back sound of the 5692 tube, also much more expensive than a 6SN7. If you want to just try a different 6SN7, try the ElectroHarmonix, pretty good overall sound and you will not break the bank. I have never used any EL34 tubes and Thorman recommends the SED which is probably a good choice, since I was going to suggest you try the SED 6550 tubes, as they are one of the best sounding 6550's. If you don't have a biasing problem, you probably might want to try KT88's instead of the 6550's. The tube complement in your amp is extraordinary and affords you a remarkable opportunity to a lifetime of tube rolling! Have Fun!!
