Popular theory (or perhaps just my own) is that there were never any major changes in the type of tubes used- from the late 60's on they had huge stocks of military grade, mostly Sylvania tubes (possibly RCA too?), from which Healy and whoever else hand-picked the best-performing ones. I'd be most interested to know if that ever varied, like the times when he switched to Mesa or GT amps. I've been using the same RCA 7025's in my DSR preamp for about 6-7 years now, which were already about 30 years old when I popped them in, and I feel like my rig sounds better than it ever has.
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia