GE, Sylvania and RCA NOS 5Y3s all work great. I just Wikipedia-ed the 5Y3 and it confirms my tests that the current Sovtek 5Y3 puts out more voltage than any of the NOS tubes. The older tubes sag more and have a greasy-er tone. Great for blues, the Sovtek might be better for a little more head room, but I like the Solid State rectifier for that. The Two Stroke never gets chimey clean anyway. NOS is getting harder to find. Any NOS from the late 1950s through the 1960s would be worth a try.
A NOS RCA black plate 6L6GC would might be the ultimate 6L6 but the gray plate versions made prior to the black plate sound great too. I’ve been using new JJ 6L6 and 12AX7s and they are great (IMO).
I have a supply of NOS 12AX7s, all in the original, unopened boxes dated 1959, but I don’t think they bring much to the party over the JJ. Has anyone tried a 12AT7?