Note that this depends on where you're setting the "Vol," dry/octave "Mix," and OC-2/Mutron "Tone." As mentioned, I've noticed the Mutron side (full CW) is a little louder (or at least more "present") than the OC-2 side (full CCW); also, I think (haven't checked lately) the overall volume may get a little louder the more sub-octave you mix in. So depending where you're setting your dials, when you hit the Sub only switch, there could be a volume and tone difference. But the "Sub Vol" knob allows you to compensate for this.
I find that, at the mix of dry/octave and OC-2/Mutron that I like, the Sub Vol knob has to be a little lower than the Vol knob to get an even volume match.
The true test is if you set the Mix full CW (sub-octave only), then hit the Sub switch, then there shouldn't be a tone change. (I haven't checked to see if the Vol and Sub Vol knobs are level-set at this point.) Anyways, I think you get the idea...