Modification to a stompbox tone control

[Marshall Arts]

I am working on an effect that has the attached configuration for a one-knob-tone control. For my taste, I would like do decrease/alter the range of the tone control, so I am looking for a good article or information what changes will have which impact on the range (without having to order, read and fully understand this: https://www.amazon.com/dp/075062986X/ref=rdr_ext_tmb).

From what I understand, this is some kind of active filter (input of course on the left side, output on the right side)...
I expect some changes from C19, R25, C20, R26, C21 and potentially the potentiometer, but don't want to try for hours poking in the dark. Any hints welcome (I could randomly change stuff on a breadboard, but I prefer a slightly more acaemic way ;-)).

[devilsnight]

As far as tone stacks go, the duncan tone stack calculator gives a good visual way to tweak what your working on.
http://www.duncanamps.com/tsc/
Also good reading here
http://monster.partyhat.co/article/amplifier-tone-stacks/

[Marshall Arts]

As far as tone stacks go, the duncan tone stack calculator gives a good visual way to tweak what your working on.
http://www.duncanamps.com/tsc/
Also good reading here
http://monster.partyhat.co/article/amplifier-tone-stacks/

Thanks - great ressources, indeed, unfortunately I don't find the circuit I have in front of me (I'd prefer to just change its values, rather than implement a new tonestack), as I am modding an existing effect circuit. If only I had a name for the specific filter that is implemented there...