You might also like this (though I don't know if it's using the R-Pi or is user-moddable): https://www.korg.com/us/products/dj/nts_2_pt/
They may still use discrete circuitry on their high end workstations, but I'm not sure. Korg embraced all-DSP synthesis early while their Japanese competitors (Roland, Yamaha, Akai) maxed out on sample playback and high polyphony. They have always had a narrow focus on synthesis, with minor incursions into the guitar utility/space, whereas Roland has a massive product catalog of electronic instruments (but a highly proprietary ethos) and Yamaha hasn't done anything in years (a pity, as they made great hardware sequencers. Akai is a special case as they basically own the market for hip-hop and its derivatives but are also unable to break out of it. I believe their gear runs on Linux, but like the other two their hardware/software stack is heavily locked down.