I too agree this is great, and see space for another low-hanging UI tweak:
Allow editing numbers to right directly, instead of only using the slider. I really dislike using sliders beyond a first general impression because it is too slow to get things exactly as I want them. I would guess this is a relatively common sentiment.
One feature I don't see but would love to see is text overlay, but that is perhaps moving away from the spirit of the project.