CV-able waveshaping, perhaps? Tiptop's Fold is a nice choice, plus it gives you some suboctave dividers to add more low-end to the drone textures. And I have to agree about the multiple attenuverter...it'll come in handy for more than just the E350, too. Happy Nerding's 3X MIA might not be a bad choice, as it can also use its three attenuverter sections as separate 2-1 mixers, and each section can also do offsets, all in 6 hp.