Hi Andy, if there's a different schematic out there that shows a tonestack, that you're thinking of, could you post a link? If that's going around, I must have missed it. I will try to give you some 'quick and dirty information' about this, although in reality there is more to it. This is enough to get experimenting, though! You certainly will not hurt a thing if you try this out, in fact, I suggest you do!
If you don't have a schematic to post, we'll start at "square one", and assume you want to just add a tone stack to the 'regular' Valvecaster, the easy way.
Enlightenment:
1) ok, if you add a tone stack, you WILL lose some volume (level) as the passive R's and C's 'eat up' signal. Actually, more than a little will be lost. That's called "insertion loss". The VC probably has enough gain to drive a stack, just that it will not give as high an output IF you add the stack just before the output. Whether the stack eats up too much level is something to be experimented with - it COULD really stink, esp. at lower voltages. I would use an FMV stack rather than Baxandall, by the way - better suited for guitar.
2) One way to avoid this is to put the stack in between the 2 gain stages. That will let the 1st stage drive the tone stack, and 'recover' lost signal in the 2nd stage (buffering is better, but beyond the scope of a quick reply). BUT - because you 'ate' some level in the stack, you won't get the overdrive you probably want!
Because of 2, I'd go with 1. You will keep the amount of overdrive you got in stage 2, just at a lower output.
BTW, The valve has little to do with whether you can add a tone stack, it is more about how you drive the stack...in fact, if set up correctly, the 12AU7 is a BETTER valve for this than the 12AX7! Typically, a cathode follower is used to drive such things, though....also called a buffer. As this is a simple 1 triode gain circuit, our options are limited unless we want to add more stages.
I would just build the VC, and make a tone stack on breadboard - don't just build it without hearing it!...connect the output to tone stack >> amp, and see what you get. This will work much better if you can increase the VC voltage, try 18V if that is possible. If the level is too low, you can then move to a bass and treble conotrol....if too low, try just a treble cut, etc. Or add a recovery stage if you really 'need' the full tone stack, your call.
If you took the time to do the math/simulate the VC in LTSpice, you can figure out the dB gain (I'd do it but no time right now)...and then see what the stack will take up, get my drift? That is why learning something like LT is so useful! If you come out with a negative, you know you're losing signal. This is a great way to learn how to mod things! Hope this was helpful....