60 Hz Hum from Powered Sub

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.

Thanks Miller! Great advice.

G

Why does that work?

If the source and preamp are on one circuit (L1) in the main, and the sub is plugged into the other side of the main (L2), the difference between the two is the hum you are hearing. Lifting the ground with a cheater removes the common difference between the two. A ground is for fault protection and a drain to ground vs YOU.

A common on the other hand completes the circuit. A common and a ground CAN (not should) share the same bus with a jumper in the main. I keep them separate. That jumper can be the source of a lot of noise in house wiring..

You can get a hum using the same side of the bus if a two component have a slight difference in voltage but share a common cable like a RCA or XLR, (good place for a ground loop eliminator). Cable boxes are notorious for it, so are laptop computer supplies.

Regards