Do you have a clock divider? I will sometimes put my VCO through a 4ms QCD and then mix the two. It does thicken the sound. A module I've debated getting is the Erica Black Double Bass. You still need a VCO to feed it, but it supposedly handles the frequency division, mixing, and filtering all in one module. I don't have any hands-on experience with it though, so I can't really say how well it works.