I asked this question over on madbean forum, since this is based on the sunking. Brian replyed with the following. Can anyone please verify that I have the mod correct in the layout, based on his reply?
From Brian:
"Take a look at the Sunking II schem. It shows to ways to increase the bass on the circuit. If you wanted to do it with a pot, I would put it where I did the "Fat" mod. Use a 50k pot to blend the large cap in parallel with the small cap. You could use different values to for more extreme results. Say 47n to 220n."