tonal adjustment on the green ringer

[12bar13]

a while ago i built a Green Ringer and i like the sound (especially in front of my FuzzFace), but i would like to have a little more versitility out of it.  how could i get some adjustment out of this thing?  idealy i would like to get a knob to deepen the tone and bring out a little more of the mids, but i don't want to lose the stock tone which has its place in my arsenal.

here is the schematic that i used from General Guitar Gadgets
http://www.generalguitargadgets.com/pdf/ggg_gro_sc.pdf

joe