I used to own the exact preamp as you have now and I remember the best tubes I had on the gain positions are Siemens E188CC with A-frame (best treble extension), and the Siemens CCa A-frame(best bass and mids).
I tried variants of Siemens E188CC and CCa that does not have the A-rame structure and they don't quite sound as good as the A-frame versions. I also tried Telefunken 6922, Ampererex 7308 white label PQ, Amperex Jan 7308's white label,Mullard 6DJ8s and 6922 BVA logo, Valvo white label E88CCs and I repeat none comes even close to the two tubes I described above. Of course this are all just my opinion on how they sound on my system back then.
I tried variants of Siemens E188CC and CCa that does not have the A-rame structure and they don't quite sound as good as the A-frame versions. I also tried Telefunken 6922, Ampererex 7308 white label PQ, Amperex Jan 7308's white label,Mullard 6DJ8s and 6922 BVA logo, Valvo white label E88CCs and I repeat none comes even close to the two tubes I described above. Of course this are all just my opinion on how they sound on my system back then.