Humming....Help

I have 2 Parasound amps which power my back four speakers in my HT; they are each 125 x 2 ; the side speakers hum appreciably; the back ones hum alot less, but still present; I have 500$ power cords in each; a recently purchased noise harvester from PsAudio.
Is this the famous "ground Loop" problem or is this a problem with the amp ?? Thanks. JCB
PS My Parasound 200 x 3 amp (fronts) does not hum at all.