- The input capacitor (C1) could be increased for more bass.
- C2 could be increased to 47-100µF for more drive.
- The output capacitor (C3) could be decrease for more treble.
- R5 could be replaced by a 10-20k pot, lug 1 going to ground. C2 could then be moved between lugs 1 and 2. This would act as a gain control (much like in a Fuzz Face).
- R3 (trimmer) could be moved off-board as an alternative bias/gain control.
I haven't tried these modifications, but they should do something interesting. YMMV