Alright, so a couple things for starters:
- 1A @ 9V is plenty to get the tubes warmed up and the circuit running
- testing tubes with a DMM will tell you almost nothing
- any modifications to the circuit should be done with the power off, so the risk of electric shock will be minimal
Regarding increasing the gain, there are a couple things that can be done. The easiest will be to get high gain tubes. The main spec you need to worry about is amplification factor, represented in datasheets as µ. 12AX7 on paper has a µ of 100, making it the highest gain tube of that series, but the manufacturing tolerance of tubes varies widely, and it's not uncommon to see brand new "good" tubes with a µ of 80, I've seen some brands (most notable Psvane) as low as 70 in a 12AX7. Unfortunately the only way to measure this is with a "curve" type tube tester like the uTracer, otherwise you can go to tube distributors like
Tube Depot and request tubes specifically tested for high gain.
Now as far as circuit modification itself, there are a few things you can try, but I will preface this by saying that you will eventually get to the point where the physical dimensions of the PCB are too small for the amount of gain, and the circuit will oscillate. I have this one in a good place where the gain is high enough to be useful, but not so high that it oscillates, so if you're testing your modified circuit and it starts squealing with the gain turned up high, you've gone too far.
That being said, here are a few places you can start for increasing gain:
- decrease the value of R11, 1MΩ is a good starting point
- decrease the value of R6, 220kΩ is a good starting point
- decrease the value of R5, 100kΩ is a good starting point
- increase the value of R3, 220kΩ is a good starting point
- increase the value of R4, 220kΩ is a good starting point
These are all theoretical modifications that have not been tested, and the starting point values are not based on anything other than me saying "yeah that's probably fine" inside my head. I wouldn't recommend going any higher than 330kΩ for plate resistors (R3 and R4) or lower than 56kΩ for the interstage resistors (R5, R6, and R11).
I will of course emphasize that you should keep your fingers out of the box when power is applied, and for this reason "guess and check" is going to be a more recommended option that swapping any of these out for a trimpot. Socketing the resistors for easy swap would be fine, but make sure the resistor leads are not going to touch anything else when they're sticking out of the socket (particularly R3 and R4).
Alternatively, you could put a simple boost like a LPB-1 in front of the pedal.
