Thanks Ecco_Luke.
Well for the theremin sound the Big Muff is by no means clean! Its a filthy dirty distortion.
At full up it creates an almost square wave from a clean guitar.
This wave form is full of high frequency stuff.
Turning down the tone knob rounds off the high harmonics created when the pedal clips or goes into distortion. This rounding off of the square wave looks and sounds more like a sine wave like a theremin.
Its still VERY distorted and best with only one or two notes at a time.
A compressor or sustainer pedal works differently and does not change the wave shape or sound of the clean guitar input.

You are right I used a very clean neutral amp setting.
I do that with all the Effectology sounds as I want the sounds heard to be from the pedals and not effected by the amp I am using.
Hope that helps a little!
Thanks again for writing.
Bill