Just a quick update to say that this project is now almost certain to use the Teensy 4.0 as its main microcontroller. The Teensy 4.0 was released this month and is somehow both incredibly cheap and amazingly powerful. I haven't managed to get hold of one in the UK yet, and the audio board hasn't yet been updated to be compatible, but all indications are that the Teensy 4.0 will be the perfect board for this project. Check out the new Teensy's specs here: https://www.pjrc.com/teensy-4-0/
I've been on a break from this project for a while, focusing on my DrumKid drum machine for the 2019 Hackaday Prize, but the deadline for that is in a few days so I'll definitely have some time to work on PolyMod 2 over the coming weeks and months. I've now got a lot more experience (albeit self-taught and probably flawed!) with PCB design and, to an extent, product design, so I'm feeling a lot more confident about making PolyMod 2 a reality.