I have owned and sold off the following filters:
Digitech bass Synth Wah - decent octave sound, but not a great filter. its a good pedal to try in a store to hear what the difference is between filter-synth-auto wah and octave but this pedal doesn't do any of these effects very well.
Mini Q Tron - This is the older large box unit. It was true bypass, simple and sounded gorgeous. But the volume boost was too much and the sensitivity drove me nuts. It had to be set just right to sound good and worked a lot better with some basses then others.
Micro Q Tron - just plain disappointing. Same big volume spike when turned on, bland sounding, no personality and no where near as good sounding as the Mini Q tron.
Ashdown filter (the purple one) - really disappointing. I could barely get the filter to open and had to max out the input which caused it to clip and distort like crazy. The VU meter was a joke, completely inaccurate and more of a gimmick. I wanted to find some reason to like this pedal as I thought the design was really neat. In the end it was simply junk.
I currently own the following:
MXR Bass Auto Q - marketed as an auto wah, its very much a filter and has a lot of good things going for it. It has a volume control, its very squelchy and prominent, and can be tweaked a lot. Only real downside is that is kind of flattens the overall tone and is considered a little bland by some. I still really like it but realize there are much better sounding filters out there. This pedal is underrated.
3 Leaf Groove Regulator. Just received this baby a few days ago. Its pretty much perfect. No volume drop or spike, great rich drippy tone, not subtle, funky and wet and just the right amount of tweakability. I will be using this pedal a lot. When a/b'd against the MXR, it retains the original tone of the bass much better and is not as nasal sounding as the MXR.
Xotic Robotalk2 - still en route. I should have it by tomorrow or Monday and will post a review. This pedal was only released this year and there isn't much in the way of bass player feedback so far. I am really itching to compare it to the groove Regulator. The feature set looks really cool with the internal dipp switches and dual chanels. We shall see!