I think I mentioned this before, but I was recovering from a hand injury and wasn't able to play bass for awhile, and messing around with all of these pedals helped get me back into shape.
I spent a good amount of time playing with envelope filters of all types today, and I realized that I have learned a lot.
There are two main components of envelope filters IMO - sound and feel/responsiveness. Some filters have great sound, but lousy feel, some have great sound and feel, some have neither. I'm finding that some filters are easy to dial in for a "good" sound, but very hard to get a great sound; I also finding (generally), that some of the filters that are harder to dial in reward you with a fantastic sound once dialed in. The feel/responsiveness varys as well - easier to dial in equals a different feel than those that are harder to dial in, and often times those that are harder to dial in have a more responsive feel. Also, most times when ppl talk about dialing a filter in, they are talking about the sound, but I think that dialing in feel is just as important.
A lot of filters will talk about getting that "old school" (i.e. MuTron) sound, but the feel/responsiveness is something different. A lot of filters cop that tone, but feel/respond differently - and that's a good or bad thing, it's just a thing.
The more I learn each filter, the better they sound and the better I am at getting the right sound and adjusting to the response. It really is like filters are an instrument themselves. I also find that the more filters I play, the less beholden I am to that MuTron sound - although it is still king of the funk IMO.
And say what you will, but components matter. The John K MuTron clone that I have uses the vintage optocouplers and components, and it is the closest to an actual MuTron when talking about sound and feel.
VCA filters (IMO/generally) tend to sound more synthy and have a different feel than optical based filters, which sound more chewy/quacky but tend to have higher peaks and need to be played with a different approach. I'd always thought that I would prefer optical based filters, but the Chunk 00 Funk blew that theory out of the water.
I also think that it's quite amusing when I see posts on the internet from folks that claim the Ibanez and Maxon filters in the same series (i.e. Ibanez AF-9 vs Maxon AF-9) sound completely different when they are literally the exact same pdal in a different enclosure. It's also funny how the Ibanez/Maxon 201 series are prices far, far higher than the next series up (AFL/AF-1) when the AF1/AFL are the exact same pedal as the 201 series, just in a different enclosure.
I'm hoping that when someone else down the road is looking for the same info that I was runs across this thread and is helpful.
And anyone reading this, I'll probably still be looking for a Maxon AF-FL Fireblade filter (even though I'm pretty sure it's exactly the same as the AF-9 inside) ;-)