T-Rex MV question

[bcmatt]

I know maybe it is a sin to suggest a MV for something that is somewhat Trainwreck related, but here's the scoop:
I'm putting together a T-Rex for my brother-in-law as a Christmas gift and it will be his very first tube amp. Cheers Richie; I believe this is your design, and it seems like it is exactly the sort of thing that he would like.

I'm still waiting for my turretboard material to come in from Turretboards.com before I officially begin.
I am using the chassis and faceplate for a Ceriatone 18 watt TMB combo.
I plan to use Dana's VVR as well on this project and I think it will be great. (will use a 3 position DPDT for standby/power to free up a hole for the VVR)
(So, I guess this amp will be a real tribute to the Hall brothers)

There is also a place on the Faceplate that is meant to be a MV for the TMB channel. I like this idea because my brother in law seemed to be into the preamp type distortion from my JCM and I am hoping that I can give him that option on this amp as well (even if he outgrows it and ends up keeping it cranked anyways-opting for the VVR option more). Then this could also give him the option of balancing clean and dirty volumes and use an A/B pedal for more versatility if he chose.

So, what do you think trying the MV in this amp? Does adding another pot suck a bit of power out of the circuit or throw off the finely tuned balance or anything? Has anyone tried that on T-Rex?
I was just thinking of throwing a 1M pot between pin #1 of V2 and the 100n cap before the phase inverter (and wired to ground). Does doing this mess with anything else? When the pot is fully cranked does it really seem like it's not even there? Or, do I need to change that 220K resister beside the 100n cap? I'm not afraid to try and play with it a bit, but I sure would love some tips. If a cranked MV pot really didn't mess with the balance of the circuitry than you think everyone would just pop one in their trainwreck clones...but they don't seem to.