Some good ideas there for bass mods. However - I'm not planning to mod this for bass myself, I only play bass for recording demos, and usually keep it clean aside from a little compression. So if anyone wants to take the bass mod ideas and run with them - be my guest! I would agree re highpass and bandpass being difficult to use with bass though - so maybe for bass this design isnt worth the extra effort - no point making a multimode filter and only ever using lowpass!
On another tack:
I mentioned in my first post about a few tiny tweaks I'm considering. Having lived with the unit for a while, I'm planning to make the 10K resistor between the Release control and ground a little bigger. This resistor sets the minimum release time. As I have it now with the 10K resistor it's too short - there is a weird low frequency hum when Attack and Release are both at minimum, so I'll try going up in small increments (22K, 47K, 68K, 100K) until its perfect. Not really a major fix, and in practice at the moment I'm just keeping the Release control a few degrees off minimum.