That's a very lucky Kiddo to have you as a mentor.  
To both of you, nice work on that base.

We are getting close to finishing project and I’m second guessing where the gain control would be added in the circuit.
I was initially thinking it replaced the 100k resistor on the output but thinking logically now realizing this is not correct. I saw in another post that Steve said the 100k pot goes on the output, but where exactly?
It seems like across the input RCA line would be the place?

Great looking build btw.

I’ve played with the pot on both sides of the zkit. I currently use on output.

On output the full 2v signal goes to the tube and you get maximum effect. Ie good harmonic distortion, and attenuation from there. The signal is “enhanced” regardless of where the pot is set.

On input you control how much of the signal the circuit sees and as such how much”effect” is in resulting signal.

I think it depends on upstream equipment ie if your amp has a pot on input, why add another series pot. Definitely not gain structuring! For my application I am feeding a solid state power amp. I want full “effect” to make this sound more Tube like, so pots on output.

I made one with 3 relay controlled inputs and a bypass switch. I found better ways to tweak my sound, but the zkit is a cool piece and I use it regularly.

I've never looked at it that way. Maybe I'll try it on the output. Mine goes from the ZKit to a balanced-to-SE transformer converter, then to my headphone amp which has a volume attenuator.

Same here. I use mine with a tube amp so I don’t necessarily need the “full effect”. It’s nice being able to adjust the drive having it on the input. Different courses for different horses.