If the hum is not coming from the speakers, and you are actually hearing the amp itself humming, then you may have a power supply going bad. I have 4 Para amps and they are dead quiet even when I put my ear right up to them. I guess it is possible that you may have some dirty power affecting the amp too but a power conditioner would fix that.
I have bought a lot of used stuff over the years and it is very common for the seller to claim "It didn't do that when I had it" so you are left out in the cold if you bought it as-is that is unless the seller will work with you.
Email Parasound and see what they say.