Quick MK III preamp tube question

How important is the v1 preamp tube to a MK III's tone? I just got my hands on two drop dead mint 12AX7A's from an old jukebox: An RCA, and a Sylvania (I think, not 100% on that one...). Would I notice much difference in swapping one of those beauties in there? I'm away from the amp right now, hence why I'm asking you lovely bunch, otherwise I'd be swapping as we speak :mrgreen:

It's an easy and fun test but can end up being a rabbit hole chasing tone ghosts that are not really there.

I've used 70's Tungsram's and mid 60's long plate Mullards in various places in my Mk3. V1 for overall tone and in V3 for the lead.

The long plate style mullard proved too bright and slightly microphonic in V1 - a feature of the long plates and other bits I believe. I tried it in v3 and it was very rich bright end - seemed more creamy gain. May depend on the design.

The Tungsram was good too and did not exhibit the over the top brightness of the Mullard.
I have old RCA (50's) power tube in another champ amp and it has a superb cranked tone. So I think old valves can have something different. Just need to find out what and where. It unlikely to be huge.

As it is, my Mark 3 now has most of its original 80's Mesa Valves back in the preamp. Sounds just peachy with those. Whatever they sourced in the 80's were good and solid.

I use the Mullard and other NOS valves in much more basic amp circuits I've made like a champ and a 5w single ended amp with a plexi preamp.

V1 & V3 are the lead channel tubes. You'll notice a change when you swap out an old tube for good new (different) ones.

I didn't like the RCA 12ax7 in the Mark III. It had good tone but it lost a lot of the aggression. If you don't play metal it would be fine. I prefer Jan/Phillips or GE in v1 and v3. Then a old Mullard 12at7 in the PI.

I've taken to trying 5751s in V1 and/or PI in a number of my Mks with some success. I think they're preferable to the 12AT7 in that they're closer to a 12AX7 in all specs, but just lower gain.