subwoofer slight hum

Is your interconnect shielded?

If you have a coaxial cable coming in, like to a set top box, you may need a ground loop isolator for it.

That's a quick fix otherwise, some questions:

• Is the sub power connector grounded?  They don't all have a ground wire.  If it's not grounded it is unlikely to be a ground loop issue and more likely a power supply issue.
• Sometimes you have the option of running XLR, which often have ground lift switches.... use that as an alternative.
• Does the problem go away if you unplug the input from the sub?

1. Two separate systems, in two different rooms:  the coax interconnect on one sub is indeed shielded, and the speaker line input on the other sub is good cables... they both have the same hum

2. The sub power is not grounded... just a two prong plug

3. no xlr option

4.  and no, even if the inputs are unplugged, the problem does not go away.  The hum kicks in as soon as the units are plugged into wall outlets.  

My hum problem was caused by a poorly shielded interconnect, once.

Just saw your last post, and think it might be the sub itself.

Bob

I am proud to use 10% of the power to watch crummy TV. 

Ancient tech.

I have 2 totally different subs. NO hum.

Yes... it is possible that it is the subs themselves. They are good overall solidly performing subs, Pinnacles, but it is certainly possible that it is the subs that hum.

It’s also possible that it’s the wiring in my 115 year old house, lol... but on the other hand, no amplifier nor receiver I've tried has displayed any hum, just the subs.

I suppose I should try a new sub by SVS or something to test the theory of the subs as hum source, and to get more modern customization, crossover adjustment and speaker integration, but aside from the slight hum which I can't hear once the music starts up, the tight musical quality of the Pinnacles is hard to fault.