I have B&W 802 Diamond mains. The PDF brochure says 90 dB sensitivity.
My noise is like a soft, medium frequency hum, not really a hiss. It seems to be mainly from the midrange driver and maybe a little from the tweeter.
I can hear it at MLP 8 ft away but it is very faint, not a big deal.
So I turned 80.3 volume from -infinity to +10 and the sound did NOT change at all.
So i hit the power button on 80.3 (turned it "off") and the sound did NOT change at all.
So I unplugged the 80.3 power cord from the Monster surge protector and the sound did NOT change at all.
So it seems to be the Parasound Halo amp (also plugged into same surge protector).
Need to more investigating. Could be my city/street has noisy power that the Parasound doesn't like.
I don't understand ground loops etc. Need to do more reading.
My Onkyo 5509 is functionally identical to the 80.3 and my speakers are 90dB sensitivity too (MK S150) but I hear absolute silence even with my ear pressed to the speaker grille.
Also, I think the guys are talking about a hiss not a hum so maybe your ground loop idea is right.
This might be a good place to start on hum issues: