my pc speakers have a loud humming. they are cheap pieces of shit.
if i move them around they get somewhat quieter (poor shielding or poor ground loop or both)
RP5's are not commonly written up for humming, so I'm virtually guessing you may have some ground loop thing going on between your pc, receptacle and speakers. (or the RP5's are bad internally..).
I'd take the RP5's to another room or another building and plug a CD player or something easy into the RP5's. If no humming at least you'll know the RP5's are fine and the problem is between the pc>soundcard>rp5>ac receptacle.
you mentioned the pc hum so maybe its thru the soundcard. That'd be my next look. Take your RP5's to a store or plug into another computer/soundcard.... Line out, of course. or try another soundcard.
anyway then you can start partitioning something else, once your RP5's are known to be ok.