Bypassing this cap makes the filter responsive to high frequency CV.
This is nice for filter FM, percussive sounds etc...
Pot to replace with a push-pull one :
- Filter "response" (or "level" if you do the VC response mod)
This mod consists in simply adding a switch between C31 capacitor's positive pole and its connection to the PCB to bypass it.
Pic : thanks to thesynthi.d