Not FOSS, but Renoise is phenomenal and works on Linux, at least. Made by a very small company. Difficult to get used to if you’re not familiar with trackers, but not that difficult. It’s not Ableton Live, but it’s way up there in terms of professionalism and flexibility. And it’s very cheap, too.
William S. Burroughs, not Christopher! Unless that’s I joke I’m missing…