A pot is a variable resistor. All you have to do is use a pot on the switch in the same place the 3.3k resistor and adjust the pot to make the preset bassy sound just like you want. This actually gives you 2 tone controls this way but one you dont adjust just set to however much or little bass you want.
You can use a mini pot and just stuff it away in the control panel or use a full sized pot asd solder it to the bottom of the vol pot and cut the shaft down. Its easier to use a mini pot.
