60 Hz Hum from Powered Sub

Try a cheater plug and see if the hum goes away!
https://www.amazon.com/GE-Polarized-Grounding-Adapter-14404/dp/B00B7PK1TC/ref=sr_1_8?dchild=1&ke...

Will do! Why does that work? Sorry, I'm an electrical noob here...

G

Is it time to just move on or is this something worth repairing? Could this be something simple, and if so, how can I diagnose it?

Probably electrolytic capacitors failing caused the 60Hz hum, you can find a good condition PS110 for less than $150!
If you want to diy, replace all electrolytic capacitors.

The 2 prongs on a plug, one is hot the other neutral, aka utility ground. The third is earth ground and is redundant. Sometimes earth ground is your hum inducing ground loop. Removing the extra ground is an easy way to improve cheap power cords and many components. Silversmith's Jeff Smith carries one around to demo and did so here at Chuxpona. 

In this case since the problem developed over time and occurs on every outlet and even when not connected to anything else this all argues for the caps going bad. Caps are cheap, the values printed right on them, making this an easy DIY fix.