I've done some more experimenting, and found two things:
1) Whatever it is, it seems to be in the ceiling over my "practice area" (aka, living room....

). No overhead lighting, but may have been at one time - maybe wiring still up there? But seems to be the only area, so far, that causes this.
B) Out of the four Thunderbirds I've tested so far (Vintage Pro, Pro 5, Blackbird, Classic Pro), the Pro 5 is the only other to pick up the hum, and it's not as loud/strong as with the VP.
Guess my solutions for now are:
A) Sit in a different spot while playing, at least with the VP.
2) Ignore the noise/pretend it's a custom distortion effect.
n) Play a different bass.
n+1) Get the owner/manager to figure out what it is & fix it....



- yeah, that's probably not a real option