I've currently a rev C GCB-100 crybaby bass wah on my workbench (belongs to Gareth Hughes of this parish). As standard it's a hopeless, tone-sucking piece of excrement. That's being polite about it. After carrying out ALL of the mods mentioned on the wah-wah site above, namely 33k Q mod (added pot for variable Q), 470 ohm gain & bass mod, 1k5 mid mod, 68k input mod, it really is rather good compared to the original product (didn't need to change the cap value as that's the only difference between a guitar crybaby and a bass crybaby). Still drops some bass end when engaged though but I plan to get round that by adding a variable blend (like an effects loop built into the pedal with the wah circuit in the loop). That should help it quite a bit I would think. Also plan on adding true bypass with status LED.
BTW did you know that the only difference between the current standard cry baby and a zakk wylde signature cry baby is 1 component value and a lick of paint... or £40 if you're daft enough to buy one new...