I have been using the Proton V1 for a few years now. Many other filters have been tried and I currently still own a couple others but the Proton stays on my main board. It just offers consistent smooth sounding, juicy filter tones that are bass-face inducing.
Like any filter, you have to practice with it to get a feel for getting the thing to open up for you but once you dial it in, it delivers.
Mine is an early model with a custom etched limited chassis... it has some limited control but I am still quite happy with it. I may try a newer version one day but I would feel like I'd be betraying an old friend.
3 Leaf Proton Envelope Filter V3
The Proton is a modern homage to the classic Mutron III envelope filter