Doesn't Rebirth connect to Reason via Rewire?
I really liked Rebirth's workflow, but l can't remember when l played on it, ages ago.
You can probably do the 303 on Subtractor / Thor, you just need the right preset sound to begin with, and l guess you will also have to artificially limit which dials and sliders you press, as the original TB303 / Rebirth's 303 had only a few dials. So l guess what l'm saying is, you've already got the 303 close at hand within Subtractor / Thor / Maelstrom etc.
But l have to admit the 303 on Rebirth was always "phat", accidental "hollow" sounds were never possible on Rebirth's 303. I wonder if it's possible to have that type of consistently phat sound on Subtractor, no matter where the sliders / dials are turned?