I'll just throw this out there, too. Lots of guys have had ground issues with their cable TV. Try removing the cable coax and see if the hum is knocked down to nothing. Also, dimmers and fluorescent lighting can definitely cause problems for us TT guys.
But, I think your single mission here should be diagnosis. I might suggest here that you start unplugging things in your AV room and see if there is an improvement. I mean it could even be your interconnects' shields are not doing their job! But, start with the basics, then move to the weird.
The Humdinger doesn't cost that much anyway, and your systems performance may benefit from it regardless of the outcome here.
BTW, one guy I know of was searching for TT hum and did discover it was coming from the transformer in his amp, and it was a loose laminate that was vibrating to the 60 cycle sinewave.