OK...but if the problem was centered around the cable/amp....it would follow the cable amp.
You said at some other locations the problem was not there...so, it may very well be the cable/amp that identifies it, bit it's something else causing it in YOUR general area.
Unless there are transmitting towers nearby or some kind of industry...the only thing it could be then is your power, or some device in the area attached to the same power your using, IMO.
Not sure how it is with your power company...but I've had them come out to my place on more than one occasion, and they check out the power, the transformers with their equipment...and once they left a device hooked up for a couple of days to monitor things so they can check if it was something intermittent.
What I'm saying is...calling them to complain of power noise/issues is usually not a band thing...most times they are willing to send someone out to check the situation.
Hey...it could be one of your neighbors with some sort of device that is dirtying up the lines for everyone around...but only your are noticing it because of the cable/amp thing. IOW...basic home power users would not notice things like that when using your typical home appliances.