Source Audio is famous for their One Series pedals with tons of options and tweakability. Spectrum is no different. It's an envelope filter Swiss armyknife with so many parameters you can adjust that you might feel bit overwhelmed at times.
The best part is that you can find plethora of funky sounds just by going through the factory presets and turning those four knobs on top of it. This pedal gives you giggles and smiles, turning your guitar (or bass) to massive late 70s funk machine. Right there in the "Mutron-like" sounds and tones.
You can enter the secondary or alternative function of those 4 knobs by pressing the tiny button on the rear panel. It takes a moment to memorize what each knob does but overally the pedal is pretty intuitive and easy to use.
You can edit your sounds with the editor software/app on your tablet or iphone. That gives you so many parameters it might take a day or two to get a hang of it. But once you get in, only the sky's the limit. You can tailor the Spectrum to fit your envelope filtering needs. It can do even some analog synth like tones.
Stereo in and out really makes this thing to pop out. I'm personally using two amps and hooking this pedal stereo really gives you a great 3 dimension filtering effect that cuts thru the mix.
If you are looking for an envelope filter but don't know which one to get (EHX Q-Tron, MuTron, MXR or any other classic one) I recommend Source Audio Spectrum. It can do all those above so well you'll definitely find your favorite sound and setting. Just bear in mind it might take a bit more tweaking and adjusting.