Parasound HCA-855 - loud hum from 3 of 5 channels :(

Though I am always a little scared buying audio gear from ebay, I have always had good luck. Until now...
So I bought a Parasound HCA-855a and it sounds and works great out of the first two channels, but as soon as I connect to any of the other 3 channels there is a very obvious and loud hum. I have totally isolated the amp, connected several different speakers, different cables, etc with no luck - definitely something internal with the amp
Any amp techs have any ideas?

Mike

Yuk - that's not cool. Is the hum coming from the speakers, or is it coming from inside the amp itself?

From whatever speaker is connected to one of those 3 channels