I tried a quite a few envelope filters before i settled on this one. There are many great ones out there, but the Enigma Q-balls for bass is absolutely ALIEN! It can tread ground that no other envelope filter does. You have 8 knobs (count em!) at your disposal to fully tweak your sweep however you want. You can pull off the classic Bootsy Mutron sound or you can get straight up aquatic belching from it. It can be a bit tricky to figure out how to set it the way you want it at first, but that's simply because it's so sensitive and versatile that little adjustments can make huge changes to the filter.
The fact that you can attach an expression pedal to control the sweep gives you one of the most versatile, customizable wah pedals out there. Pretty great feature which i use A LOT.
Adding on the analog distortion switch is the icing on the cake. It's very subtle but can add a super pleasing crunchiness to the filter. preeeeetty sweet!
The only bad part about the pedal is how extremely loud it can get if you set the knobs wrong, and that can take time to figure out. But the way i see it, that's just a symptom of having something as flexible and tweakable as the Q-Balls. The fact that it can get that crazy means you have that much headroom to work with. A necessary evil, if you will.
Electro-harmonix Enigma Q-balls For Bass
Highly flexible, shapable envelope filter tailored for bass guitar