I believe that the gain is determined by the value of R4. The GAIN pot is acting more like a volume pot to control the amount of signal passing to the next stage.
The MID response of the tone section is determined by the values of C11 (1n) and the mid pot B500K. The hi pass filter produced varies from 265 Hz at full CW to 319 Hz at full CCW. You can increase the range of this filter by changing the C11 to 1n5. This will lower the range from 177Hz to 212Hz. You should then hear more mids along with more interaction with the bass control. I hope this helps.