Ok, so V1 feeds both channels. Originally you wanted to lower the gain in the OD channel. NORMALLY, V2 would be the tube I'd look at first for reducing gain in the OD channel and that has a good reason. In many amps, the first gain stages directly related to the OD channel, but NOT the input gain for everything (V1) is a good place to start. And with most amps, it works best. But multiple Archon and MT15 users have tried the lower gain tube in V2 and V3 and so far all of them said that V3 sounded better. Without a schematic and an analysis of the circuit, I can't say why that would be true, but so far everyone says it is.
#2- I'm a big fan of using a boost in front of a tube amp unless the amp is really cranked up. I've owned a lot of tube amps, and at even medium volumes, a boost seems to make the amp sound better. So unless it's some classic single channel Fender amp that is overdriving when cranked and I"m trying to lower the gain level overall, I never replace V1 with a lower gain tube. Oh, I've tried it. But in every case it defeated the purpose of what I just mentioned. Lowering the input gain made it even more important to use a boost (for me, and for the better tones it gave) and ... to crank the boost up more to compensate. At which point you go, "WHY?"
#3- I don't have this in front of me, but not every tube with a 12 in front is identical in it's electrical properties and just varies in gain from the others. I've read a detailed analysis of why the 5751 is the best tube to replace a AX7 with if you want to lower gain, and it is NOT just because SRV did so. It has to do with voltages it runs, etc. Some manufactures will even void your warranty for running certain AY or AT tubes. In some designs, it throws things off enough to make them a little unstable, but in some it can actually cause damage. IF I find this, I"ll link it later.
I'm glad that what you tried is working for you. I still haven't gotten around to trying my NOS 5751 in V3 of my Archon, but I've never felt the need to do so. I can turn the gain down way low and get classic rock crunch easily without giving up anything. And I while I would never say I wouldn't even try it in V1, I'd be shocked if the amp was better on both channels with it there.