I have a Rocktron Heart Attack, but it's easier to say Bassballs, because people already know what I am talking about. I would like to see if I could mod it for an expression pedal to control...something? Sensitivity, filter sweep, maybe? What one of those would even be practical? What else can you stick things in to control in that circuit?
FWIW, I really the dig the bass sound on a lot of Church of Misery's albums, and I know that a filter is used. The sound I like starts in at about 0:51 in this video:
http://www.youtube.com/watch?v=AqcWyc1V4c8
It's probably a wah pedal, but if I can easily mod a Rocktron heart attack (it has a fairly simple PC board in it. I was surprised. No SMDs at all!

) I will post pics of the board if anyone needs them.
Also: maybe a control knob for the distortion? or some/more clipping diodes or something to make the disotrtion sound different?
I'm sorry, I know this sounds ludacris, but I can't help it! Once my brain starts going on something it doesn't stop....