What I already have done (and liked) :
- A switch to choose between two caps in the entery of the circuit, for exemple, a 0.22uf and a 2,2nf.
- a bass cut knob in the end of the circuit. (http://www.mylespaul.com/threads/bass-and-tone-cut-controls-i-may-never-look-back.327091/
). It really sounds great and I really like the fact to control the quantity of bass I cut. I'm not sure I would put it in front of the circuit.
For a low pass filter, i'm experimenting caps between the base and the collector of lil'trannies. A good way to reduce the hiss...