Sorry I missed this one.
The 220nF cap allows more bass frequency than can be used (with a Bass guitar) so the only real mod would be to switch that value to tighten that up when using for standard guitar which is 100n and even 22n or whatever in between pleases you. You won’t necessarily hear more Bass unless using a Bass guitar but you will be choking off Bass for a tighter, brighter guitar by moving towards 22n. That is why we have a SW1 on the Stage 3 to switch between these values at C2 and C3.
That said with a Passive Tone you can never really add Bass but you can allow more or less Bass Frequency to pass through.