You might try a 6DJ8 variant which was specified for the high end of its day: Tektronix, HP, German Post Office. By far the best small signal tubes I've ever heard were Telefunken 6922's, specced for HP test equipment, the stuff that put men on the moon (1962 vintage, I believe). When I was collecting, very few people understood what an HP designation really meant, and they commanded no premium at all !!!!
Upgrading tubes
Recently purchased a new Vincent Audio SV237 integrated and the stock tubes are barely broken in but can’t help but think I can make it better by upgrading the tubes. Currently it has 2 6N1P tubes and 1 12AX7 tube that are chinese manufactured. Any recommendations for upgrading and matching a better set of tubes from different manufacturers?