Let me explain how the gain of an op amp works for an inverting amplifier. The 2nd stage op amp TL074-B is an inverting amplifier with the gain equal to the feedback resistor (CW1 the 250k pot) divided by the input resistor R14. So with the pot set at 250k the gain is equal to 250k divided by 27k equal to -9.3 if you figure that a strummed cord is 1 V rms (2.8 V p to p) the output will be 26 V p to p just within the supply rails of the op amp ( +15/-15). The minimum gain if you do not put a resistor in series with the 250k pot is 0/27k ohms or zero that is why I recommended to put say 10k ohms (27k would give you unity -1) in series with the pot so you wont have to guess where to set the gain pot if you don’t want to use it. Where are going to put the volume pot on the chassis with this mod?

Regarding the value of R17 I retested the amplifier after adding the reverb looking at the cathode current vs voltage across the EL84 found the cathode current to high giving 14 watts on the plate of the tube so I backed off that value and wound up with a slight tweak to the stock value. It looks that the stock value is not so bad. What I do have now is a 150 pf cap in parallel with R7 (I might try to lower this value to 100-50pf) the reverb required this to prevent feedback and this may have cured the oscillation problem I saw before.