I use fm8 for my sub basses, add a little bit of feedback, crank the voicing, then compress it and roll it off at about 1khz. Also pitch bend automation is important too, if you have a hectic growl with some crazy pitch, you want your sub to copy it exactly. before you bounce it to audio (im talking from an ableton uses point of view) export the MIDI clip because it also exports the pitch automation in the envelope, then when it comes to adding your sub bass pitch automation it makes your life a hell of a lot easier.. hope that helped