For bass modification, C1, C5 and C7 all filter bass.
You could double C5 and C7 to 200/220n then switch C1 for more/less filtering at the input.
Or do a filter cap pot where you blend between 2 filter caps with a low value linear pot. If you try that, experiment on the breadboard. Try values that are at least 10x each other, 47n and 470n or even more 20n and 1u.
I'm toying with this on a muff build but wasn't happy with my values and haven't circled back to it. You need to exaggerate the values as they'll always be in some amount of parallel.
Switching is much simpler. Or even a rotary.
Here's one of many threads on input cap blending
Input capacitor blend math
www.diystompboxes.com
Some over at madbean as well
Variable Input Cap for Fuzz Question
www.madbeanpedals.com
Good luck!