How to stop audio from "warbling" when using SoundFlower
Apparently the issue may lie in SoundFlowerBed, not SoundFlower itself.  But since SFB is an abandoned project... it is unlikely to ever be made more robust.

Fortunately I think I've found a solution, which is Sound Siphon.  I'm using it exactly as I would have with SoundFlower - routing my system sound and AirFoil to Sound Siphon, and sending Sound Siphon out to my audio interface.  

Seems to be working perfectly, no audio warbling, and I can still adjust my overall system volume with the usual OS controls.  Plus, the demo version allows unlimited use of System Audio, which is all I was after... so this solution is even free!