Hi Adam,
Thanks for the updates.
I mainly use my mics outside, feeding a portable recorder with XLR inputs. I also currently have two PUI AOM5024, wired as simpleP48, "permanently"mounted outside, in our back yard, with about 40 ft (12 meters) of inexpensive shielded balanced wire (similar to your West Penn wire) running in our house to a USB audio interface that has phantom-powered balanced mic inputs. I haven't encountered any AC hum, or other interference in any of those situations.
This is what I suggest: make more tests with your simpleP48 mics, in different locations, to get a better idea of what circumstances lead to hum. Also try different devices if you can, like other recorders or interfaces, and borrow some commercial balanced mics, including ones with and without transformers, and try them in the same location, over the same wire, to see if they also have hum issues. Theaters can be difficult environments because of electronic noise from light dimmers.
And how long were your mic cables in the theater situation that gave you hum?? The simpleP48 arrangement is surprisingly good, but it's not perfectly, symmetrically balanced, so it might be more susceptible to interference in difficult situations, and/or runs of over 10 m.
