This is super impressive. I have a tiny bit of (hopefully constructive) criticism though, and it's not really against this work and what has been accomplished.

The TB303 has one of the worst sequencing interfaces of any hardware sequencer, so even as much as it is loved, I'd love to see something more modern in terms of UI for sequencing melodic parts.

Even step select/activation with 16 steps and keeping the TB303 keyboard sort of interface for actually selecting notes would be a lot more usable and immediate. Having to select the step number and enter a note is painful though and not very mouse friendly.. with hardware at least you can mash the heck out of the button.