personally id only use the headphones out for the vcf in (distortion) :) But for a cleaner polysix sound one could disable/bypass the LM13600 op-amp that acts like a sort of a low pass filter, its on the effects board and can be (un)done with a fairly simple hack and non-destructive mod, just look it up here: http://synthhacker.blogspot.nl/2013/07/polysix-bypassing-post-effects-vcf.html