Well if the circuitry supporting the 3396's is designed to use pots, then an analog interface could quickly be attached to it (just wire up the pots), or digipots could be substituted for a digital interface, which could then use pots or encoders. I do think it's best to choose one method or the project can become too scattered, but this way doesn't create a branching off of the project, but a split, into the synth circuitry around a 3396, and the CS. I'm going to go read the datasheet again, because I obviously missed the part with the 4051's... I'm all for following the datasheet, I am so clueless at circuitry.