Robin, hopefully some other folks will chime in, but my experience with the filter knob (which only affects the octave voicing, not clean or either of the synths) is thus. At about 1:30, you get the 8-bit NES type sound and as you move counter clockwise it gets "lower fi." Above about 1:30 or 2:00, it gets synthy in a different way than the other two synth voicings. So if I want to solo the "clean" octave voice, I turn to about 1:30 or 2:00, and if I want OC2 type tone, I go with 11:00 to 12:00 plus the clean voice.
I should definitely point out that I'm AT LEAST the THIRD owner of my specific pedal. The previous owner indicated that my pedal was sent back for the upgrade that you'll read about in the early pages of this thread, but I wouldn't rule out the possibility that this is not actually the case. Upgrade or no, I totally dig the pedal as is, but if it doesn't actually have the upgrade, your experience will be different.
As a final thought, a little goes a long way when it comes to the two synth voicings. For me, anything more than about 9:00 is too much. But I have to admit, I haven't run the SubT through my envelope filter (in need of repair) yet.