You can still find NOS (new old stock) and ANOS (almost new old stock) 6V6s for decent prices. There were about a kabillion 6V6s made during the vacuum tube era. There are a lot of good used, but not used up, ones out there. Try to locate a pair of old American made 6V6s first. If that fails, then I recommend to go for the TAD (Tube Amp Doctor) current production 6V6s based on my experience. I had a pair in a 6V6 amp and they worked good.
An ANOS or NOS 12AX7 (ECC83) is also a good idea in V1. The current production Tung Sol brand would be fine in V1 if you can't find a suitable old stock tube. The Mesa branded Chinese made 12AX7s usually work good in gain positions like V1 as well. V2 and V3 need not be old stock tubes. Some of the current Russian brands (Mullard, Tung Sol, EH,) sometimes don't do well in cathode follower positions because of the way the filament is spiraled. This is usually V2 in older Marshalls. The Sovtek long plate called the LPS is popular in V3 or phase inverter positions. I don't like the JJ ECC83 in V1 of Marshalls but you may like it fine. Actually the common Chinese 12AX7 usually sounds good (really fat but open) in Marshalls. TAD, Ruby, Mesa Boogie, ect.... test these and then put their logos on the ones that pass the test.
