I updated my stereo sound testbed. You can now modify the rolloff, listener depth, distance model.
Maybe it will be useful for people who want to test out the effects of each of these settings.
I'm trying to find a sweet spot where sounds on-screen have a small amount of left/right difference, but then more when they go off-screen.
I think I'll fake this by reducing the overall volume in a separate routine that is applied once the sound source is off-screen.