The sub should have come with a three-prong plug. It would prevent some problems, IMO.
It should but it can also be a source of noise if there is something dumping through the same ground your amp is using. Like a 220/240 single phase motor on a heat
pump, dryer or stove. Just try it. I've had to do it on valve amps behind noise. They went from having a lot of floor noise to ZERO. IF not having a ground was such a
bad thing why did they do it (and still do). It's a safety ground, but not to complete a circuit. It's usually to prevent over voltage from a chassis if there is a problem.
I still suspect there is a cable issue and shielding or like I said a 220/240 causing noise through that ground wire. What could have happened at the other home is
THEY weren't using the ground prong or they didn't have a problem with any of the 220/240 VAC components being used in that home or they had good shielded
cables to begin with.
Again the RCAs and PC can be a source of noise. BUT the cheater got rid of the noise. Going backwards, you can still hunt down the noise.
The crap I've gone through with RCAs in 12/24VDC in heavy equipment would blow your mind. It was always a 5.00 USD RCA aluminum/copper clad cable
in a 300-500K piece of machinery and a 15.00 usd COPPER cable from PE fixed it every single time. The sub-power amps wouldn't activate unless you turned the
volume UP. Trust me, operators like their music once they get it in the cab.

Aluminum and stereos just don't work well together in some cases. I hate
the $hit.
I've seen cable routing cause all kinds of noise problems, too. I usually just pick up the run of cables one at a time or the complete bundle and see if
the noise yoyos. You know goes up and down. It's a VERY good chance, careful routing will get rid of the problem. I'm one of those guys that HEAVY
shielding is not my preference. It can really affect the quality of the sound, it's like throwing a blanket over a speaker in some cases.
I've seen power conditioners do exactly that, to a power amp. Plug them directly into a good dedicated circuit and it's quite the difference.
I just went through a houseboat with a buddy recently that was plagued with floor noise, it was the neighbor's 1000 year old freezer that ran on
220/240, they were on the same 220/240 dock plug. I suppose someone could chime in about why it happens, I'm just a mechanic, I'm pretty good
at troubleshooting any kind of noise, even a moaning neighbor at 03:00 in the morning. I told the GUY to use a sock, while I put my muffs on.
These are just suggestions, and things I've personally experienced. I throw them out and if something sticks, GOOD if not, it sure didn't cost anything.
In any case, I need to feed the chickens, speaking of making noise.
Again happy hunting.
By all means, let us know.