I have quite a few nos tubes here and quite a few current production tubes here to try out. Seems I have changed tubes in my mark III more times than I care to imagine. Right now I am very happy with what I am running and I think the combination makes sense with this amp.
This is what I'm using now
v1 - nos rca blackplate 5751 -tried this tube in different amps, different spots without being very impressed until I tried it here with the combination of other tubes it sounds stellar!
v2 - nos tungsol 12ax7 love this tube, probably works better in more applications than any other tube I have ever used. A great choice for v2, if I had another one I'd probably use it for v4.
v3 - nos mullard 12ax7 i62 -great tube for the overdrive spot, rft 12ax7 is another favorite I like for here.
v4 - mesa 12ax7 -perfectly adequate for the reverb spot, at least for me since I rarely use the reverb lol!
v5 - sovtek 12ax7lp -I have tried tons of tubes in the PI spot and keep coming back to this one. a 12at7 is another option but kinda changes the feel of the amp, might be preferred by some but the sovtek is the one I like with this combination.
I know nos tubes can get expensive and not everyone likes using them. Personally I love them, and they seem to sound much better than most current tubes. If i was on a tight budget I would at least start with the sovtek 12ax7lp (current production) for v5, pick up a mullard or rft for v3, and try to score a nice nos tube for v1 when I could afford one.