Based on what I read in this thread and 3 Leaf's YouTube clips, I recently picked up a Proton pedal and I gotta say I'm pretty pleased with it -- it has a great tone (as pretty much everyone in this thread has mentioned).
I did email the developer, Spencer, with the 9V vs. 18V question, and he said, "It's a subtle difference that's hard to describe. You'll have to try it for yourself and see what you think. I run mine at 12v."
So...take that for whatever it's worth, I guess. I haven't had the opportunity to run mine at anything other than 9V yet, but I hope to dig up the right power supply soon. Or maybe I'll just try it with a pair of 9V batteries