The Tiptop Fold is definitely something to consider, but I'd be more inclined to use it on the modules since the MB2 already has its own waveshaping via the "Brute Factor" control. One thing you might seriously think about, though, would be something more potent as a second VCO to tandem with the STO. Not only would this give you the ability to create nice, thick, detuned sounds between the two VCOs, you can use the dual inputs of the Fold as a de facto mixer, bringing them directly into the waveshaper. My suggestion would be to look at the Doepfer A-110-6, as this would also add thru-zero FM possibilities and the modulated timbral capabilities that brings. Or for around the same price, you could even go with the VOID Gravitational Waves, which would put something more West Coast-ish in with a complex VCO to team with the STO.