Ok, the trick is to really over drive the filter. What I do is to take the headphone out, run it through a graphic fuzz box and back into the audio in. Then carefully raise the external volume/gain. You will want graphic EQ in there because some frequencies when driven to hard make the filter 'bump' into extreme overload/resonance, and you may want to tame that sound because it is frequency dependent hence some keys may come out like a clang. Depending on how much dirt you want in it, I like it subtle and turn the audio mod up a bit and you get that vcf crackle sound of a Moog, which I liken to the sound of fire.