OK well you're lucky I have nothing more important to do today:
Had and sold:
Mini Qtron
One of my first pedals. I didn't really know how to use it at the time so would like to try one again. I liked the design and seem to remember getting a good sound from it with my Mexi Jazz bass. However one thing which I didn't like was that when the peak of the filter is reached it gave a slight whistling sound which was hard to remove. I felt it responded a bit unevenly so that bass frequencies would slam the filter open whereas treble wouldn't produce enough umph. Probably partly due to my technique and not knowing this at the time.
FX25 (sparkle)
I really liked the FX25. Like everyone else, I head it on the Flea video. In complete contrast to the Qtron, when the filter was slammed open, the sound made a lovely squelchy growl. Sounded best on a Stingray. Used it live quite a lot but had to have a limiter afterwards to take the massive resonant spoke in the bass end as the filter closed. A bit difficult to control because of that.
MXR M87
Had a couple of these because I really wanted to like it. I believe it's a band pass with a useful blend control, so it worked well for accentuating slap on a jazz bass but didn't find it thick enough overall. I do like the band pass sound but I really didn't like the sound of the 'Q' control which added a rather odd sounding vowel like character to the response. It seemed quite dependent on the chosen bass and suited modern actives better, I think.
3Leaf Wonderlove
Nice bubbly underwater sound, but I recall having trouble using it in a mix. I didn't use the expression input or the loop much. I think I found the range switch a bit limiting, with neither feeling like the sweet spot. Maybe it was a bit peaky sounding, kinda thin, I can't really remember but I think my biggest issue was that it needed a power supply, and at the time I was pretty much using pedals with batteries (sans pedalboard)
3Leaf Proton V4
Didn't like this one at all, to be honest. Found it to be quite sterile and cold, somehow felt a bit uninspired by it. I think technically it was good, but maybe I was expecting more, or it was over hyped. May feel differently if I tried one again. Maybe I had a bad week or didn't like the colour (actually I think it looked nice) but again it needed a PSU
Meridian Funk-u-lator
Loved the vibe of this pedal, and the synth/octave side especially sounded awesome. I lost a bit of faith in it because I had to send it off for a repair, which the designer did for me personally (and I think free of charge) which was brilliant, but still. I found the envelope sound hard to get right when used on it's own. I didn't think the controls (range, attack, wha) acted as I would expect, so it was hard to dial up a fat filter sound - a bit like the MXR I think it excelled at some band pass filter on top of a dry signal as opposed to an all encompassing filtered sound.
Mr Black Fwonkbeta
Very thick sounding, but hard to get the treble and bass behaving well with the basses I had at the time. I think the 'Fwonk' control was a bit like the Q control on the MXR, it added a vowel like sound which I didn't like personally, and other than that the controls were limited. Shallow maybe but I wasn't a fan of the graphics or the slightly cringe marketing. I liked the LED jewell.
Aguilar Filter Twin
Quite liked this one. Fan of the construction and feel, even the colour. Quite unique sounding, a bit like the bassballs having two filters overlapping, but in comparison to the Bassballs I missed having the throaty distortion added to that. Probably would have kept this but must have sold it to buy something else. Would like to try again. I think it would have been more useful to me if it was a single envelope with more control.
Chunk Systems Octavius Squeezer
Very complicated and full of useful sounds. Slightly outdated and obsolete but in the way that I like - I'm always fond of really unique and left of field products which serve a nice within a niche. The squeezer was great and I had fun programming in sounds, menu frustrations aside. I found it a bit noisy though (may have been my use case) and probably didn't dig into the sounds enough because at the time I wasn't using a great deal of fuzz.
Maxon AF9
I loved everything about this one; the size, the sound (pretty much exactly like a mutron). However I found there wasn't enough range in the sensitivity control; some basses had to have that almost maxed out which was a problem particularly with the down filter. I love the little mini switches (mine was the two know non-slider version). Hard to track down - I should have probably kept it and maybe checked if the sensitivity could be adjusted.
Lovetone Meatball
This feels like a little bit of everything rolled into one, except from having two filters active at once. I'm divided on the construction - I like the size and format, but I have Valco made pedals with the same enclosure which have been improved slightly over the Lovetone (locking latch, rubber feet). The controls are quite sensitive/easy to turn so unless I get some locking knobs it may make more sense to keep this as a studio piece rather than for use live, but as I get to know it I'll be able to dial in things fairly quickly. It has the best bits of the resonance from the 3leaf, the warmth of the Mutron and the useful blend control, true bypass and status LED. It's in the sold category because I bought and sold two - the first one had a broken decay knob, which the seller realised when he was packing it. I gave it a go on the understanding that I could send it back if it wasn't easy to fix, and after a couple of emails and a struggle to find anyone who could look at it this side of my death I sent it back. Bloke was cool about it though and it was great to try. Second one had a constant warbling/oscillating so I sent that back too, bloke was a bit miffed and said it didn't do it at his house, so it remains a mystery. But it definitely didn't work at mine.
Solid Gold FX Supa Funk
Wasn't a fan of this, I returned it quite quickly. Just didn't work for me, and not what I was expecting. It was much more in the bassballs camp but with a drier sounding fuzz which didn't compare favourably at the time. I think I might give it another chance now though. I think dual filters are very different effects to their single filter brethren and I maybe didn't acknowledge this at the time, so I was trying to get something completely different from it.
Mutron III
Had to be tried. Didn't like the very leaky bypass and the fact that the gain control for the filter increased the level of the effect. Thankfully with most of my instruments the gain was right at unity. Odd power requirements meant it took up two slots on my supply, but happily batteries seemed to last quite a long time. I love having an off switch which means you can leave it pugged in without any battery drain. Don't know why that's not more common. The sound was great - very wet and liquid, without ever sounding harsh. Pushed to extremes it remained musical and usable, though I didn't use it like that often.
Qtron (big box)
Had a dodgy footswitch but sounded great. Didn't feel particularly sturdy but I'm sure it was. In an A/B comparison with the Mutron it lost out by a narrow margin, but gained points for having a very good (true?) bypass and stays LED. Slightly awkward power requirement (24v I think)
Qtron+
As above, but with the useful addition of an fx loop. Especially liked the graphics on this one but still lacking the construction - mind you, next to a Mutron, even a solid block of steel feels a bit weedy). Sounded actually quite different to the standard Qtron - with added response speed control. The mix setting never worked as well as I hoped it would in my head.
FX25 (light green V1)
Same as the FX25 higher up, but in a rarer colour. It sounded slightly different to the sparkly one, I think better but as is often the case I may be just assuming that because it was more money. I bought and sold several FX25s and was determined to keep this one, but in the end the need to have a limiter after the pedal meant it didn't fit in my setup and a bit too expensive to have sitting pretty on a shelf. I still maintain that the sound of an FX 25 being pushed wide open has a unique sound which nothing else does.
Xotic Robotalk 2
Rather like two FX25s in a smaller housing, with different settings for each and without the resonant spike. Really liked this one, and now I'm not 100% sure why I sold it. Perhaps it didn't have quite the smoothness of my other filters at the time (Mutron). Looking back I used to be really sad and analyse these things late into the night, niggling over the smallest things. I need to grow up. Actually, no. You do.
Currently have:
Bassballs Green Russian (V1)
One of my first pedals along with the Mini Qtron. Well not this exact one but a USA version. At the time I found it crazy sounding and didn't keep it for long. Since then I've come to appreciate them a lot, finding with this model in particular that a low sensitivity and palm muting can give a really fat synth sound, a bit like every effects pedal you own turned on at once run through a phaser. Quite an epiphany using it in a band mix where it just... works. This green one I bought simply because of it rarity, but I genuinely do think it sounds the best and I just love that massive ruskie footswitch that goes 'kerchunk'. Adjusting the sweep of the filters using the internal trim pots helps you perfect the response to your instrument (something Mooer have added as external controls on their Sweeper)
Bassballs Black Russian (V1)
Obtained a black paint version of the above in the rare tall case. Sounds slightly more 'robotic' than the green one, otherwise the same applies. A keeper for sure, and nice to have a standalone one off my pedalboard if I want to use it on its own, or use it for home defence.
Bassballs USA
Used to be a bit fan of this one - sounds even more robotic than the black one above. Certain basses suit it better than others, I keep it around as a reference and because they're quite cheap.
Mutron III+
Was fully expecting to hate this, having read everyone slagging them off on Talkbass, partly due to the apparent bad blood between the inventor of the original pedal and the designer of the new one who was an employee. Personally not affected by that as I don't own shares so I just wanted to see how bad it was. Despite many threads about it, actual comparisons between the original III and the new III+, by two people owning them both at the same time were almost impossible to find, and several people had seemingly made their mind up despite not having tried both, at least not at the same time. So I made it my mission to do so, and you know what - for the sound I settled on with original my Mutron III, I was able to match it perfectly on the III+. Not every bass worked well with the III+ but my main instrument sounded even slightly better with the III+. I also prefer the power requirements (18-25v) and the true bypass. The internal dip switches allow the response of the up and down to be adjusted somewhat and I don't know what to say, it just suits my bass and playing perfectly. Even flipping the switch means the down filter is dialled in without having to adjust the pedal. I think this pedal exemplifies how viscous internet rumour and anti digital bias can cloud judgement.
Crowther Prunes & Custard
I categorise this as an envelope filter but it's kind of different, in the same way as a Bassballs is. It's sort of its own thing, but it has an envelope sensitivity which is great fun. Best when used in a subtle way, but it can get quite nasty. I dropped this as a live pedal as I felt it got lost in the mix more than the Bassballs.
Lovetone Meatball
Third time lucky? So far I have matched the sound of the Mutron III+ and have plenty to explore either side, up and down around that. Very happy so far and debating whether it'll replace the III+ on my pedalboard, but even though I'm used to big pedals even I am wondering if that's wise.
So there you go. I hope you're more impressed than my boss who thinks I've been absolutely smashing my TPS reports today