Hey.
Today I played with this pedal, I did a lot of mods, I share the most interesting.
1) Tone control.
All components are connected not to the VR but to the ground, this increases the range of work (in the case when connected to half-power, the diapason was very small, almost invisible).
2) C6 can be connected to the ground instead of VR, the sound becomes slightly more aggressive (just a little) and colder, but the board is more convenient to breed.
3) C10 in the process is useless, you can remove it or replace it with a smaller value 47p-100p to remove the noise of the operational amplifier
4) R4 increase to 2M2 this will give you an input resistance of 1M1, instead of 687K
5) R18 can be removed - this will give a noticeable increase in volume and gain (use the toggle switch)
6) with clipping modes is not particularly worth experimenting with
I tried everything (silicon, germanium, mosfets, soft and hard, symmetry and asymmetry) an optical variant in prince of tone
7) operational amplifiers tl072 / 082 / lf353 cold and uniform sound
JRC4558D highlighted middle of a little bass and treble
ne5532 / LM833 is the golden mean, 833 is hardly midst 5532 is the most natural.