Ok, just wanted to make sure, but yes that is the correct tube chart and mapping of their gain stages.
So, in order...
V1 is always the "first gain stage." Being called the "input" tube here doesn't change that. In many amps, THIS is the first place you look to lower gain on a gain channel of an amp, since reducing it in the very first stage just lowers the gain all the way through the circuit. There are a couple things to remember here. First, when you do that in V1, you're also lowering the gain to BOTH channels. You may not want to lower the gain of the clean channel, but doing it in V1 will do that. Second, remember some amps only have 2 or 3 preamp tubes, so in this simpler circuit, V1 can be a better place to start with lowering gain. But with the more gain stages you add, the more flexibility you have to do something that only affects the gain channel, and not both channels. Again, you may not want to reduce the gain of the clean channel and 5751 in V1 will do that!
So initially, I mentioned that V2 might be a great place to try a 5751 in this circuit as it is the first "gain channel only" gain stages in the OD channel. Several people tried this, including my boy Bodia, and they all seemed to agree that is worked better in V3 than in V2. I am honestly not sure why this is, and since I like my Archon as it is, I've never tried to to confirm that I agree with that, but everyone that tried it did so that's good enough for me. I have not done a complete circuit analysis on the Archon gain channel, so I'm not sure why it seems to work better in V3 than V2, but everyone that tried it said that it did.
As for would adding another 5751 be "more of a good thing" or going too far, remember this. Each preamp tube has TWO gain stages. If I remember correctly when I saw a schematic for the Archon V1 uses one side for the clean channel and one side for the gain channel, so only 1 gain stage for each channel. I won't swear to this but am pretty sure I remember that correctly. From there, we'll say V1-1 goes to the next clean channel gain stage(s) which is both sides of V4, so two more gain stages. Meanwhile, we'll call it V1-2 goes to V2 and V3, so 4 more gain stages for a total of 5. I only said that to clarify my point. When you drop a 5751 in V2 or in V3, you've already reduced the gain in 2 gain stages out of the 4, (5 total including input tube). So IMO, a 5751 in V2 and V3 WOULD probably be way too much gain reduction... it might work. It might even sound good. I'd just expect that it would be over doing it and I wouldn't expect it to be as good. Plus, one 5751, by reducing the gain from a scale of 100 for 12ax7 to 70 for 5751, and doubling that, you've cut the power to TWO gain stages by 30% each. IMO, that should be enough to tame even the mighty Archon enough for smooth fat bluesy tones. I'm sure it will still get gainy if you turn it up, but you have SO much more range before noon on the gain knob that it should be... maybe perfect.
As for the very last question, you're not going to get the same effects with the tone controls. Use those for tone! However, PRS has tremendously increased the "range" of this amp by including the presence and resonance (depth) controls. You can really shape the amp with those within the realm of any tones you've already dialed with the tone controls. So, IMO, for much rock you want tight, punchy, fast, etc so presence up, depth down. IMO, for blues you want the opposite. fat and smooth. So, presence down, depth up some. This will cut some brightness but also some punch, and dialing the depth up will loosen up the bottom end some for more fat smooth more rounded punch. These controls will be speaker/cab dependent but again, having the depth knob allows you to dial it to match what you have.
So hopefully this gets you WAY into great blues tones. Oh, one more thing. NO V30s! For blues, you want mid power alnico (best) or mid powered Greenback type (also great) with this type amp. V30s are too spiky in an area you don't want it. The other brands (Warehouse and Eminence) v30s may be great though. Emi Red Fang Alnico SOLID GOLD! Warehouse ET65- great!
