Here’s exactly what SolidGoldFX said: “The LPF circuit is a passive circuit on the output that is attenuating a portion of the high frequencies and it is designed to be a more subtle enhancer rather than a heavy handed active filter. Depending on the size of the capacitor we use, the ear will perceive a slight mid bump if we're only cutting down to 2-3kHz or scoop if we dip down lower. The LPF circuit is bypassed with the toggle in the center position.”
So, I think perceived is the key word here and focusing on the mid bump or scoop is a bit of a red herring.
Position 0 disconnects the LPF, and the sound is louder and brighter. Also more clipping (overdrive).
Position 1 softens the sound, takes out some treble. Like a bass’s passive tone knob it darkens more counterclockwise. However I don’t hear a mid bump, really.
Position 2 has a stronger effect, softens the sound even more, and more as the knob goes CCW. It takes out more into the mids, so it really does sound quite similar to a mid scoop, similar to the contour switch on a Rumble. It’s not quite the same sound though, leaving a little more mid and less treble.
What the contour on a Rumble does, a big notch at 700 Hz (blue line):