Question about hum from speakers

I know there's at least one amp engineer here... I'd like some help.  I bought a used solid-state amp from ebay; the ad read " McCORMACK POWER DRIVE DNA-.5 Deluxe REV B POWER AMPLIFIER - EXCELLENT CONDITION!"
Well, I swapped it in, replacing a Dynaco Stereo 120, and there's a hum from both speakers.  I have to believe that the seller was well aware of the hum.  What I'm wondering is, is the hum a result of some electronic component(s) in the amp failing (or having failed), such that it's salvageable by replacing the bad component(s)?  Or would that be hoping too much?
All great suggestions. One other possibility could be too high gain combination between the preamp and the amp. My (tube) preamp has a separate output gain control - in addition to volume control, and I can increase the gain to the point where I can hear buzz/hum from the speakers. Preamp gain and amp input sensitivity mismatch can cause this too if there's even the slightest ground noise.  
Hum like you're getting now is caused by different ground potentials. In other words one component path to ground has a little bit more resistance than another. Usually simply plugging everything into the same outlet (power conditioner) will do. But don't knock yourself out, not when you already have a fix. And you can forget about the fire, death and destruction that will supposedly happen if you keep using the cheater plug. Or you can disregard this post, as it comes from a guy who has already been electrocuted to death by soldering on a lethal voltage tube amp.
If the cheater plug works try the HUM-X device. It goes between the AC plug and outlet and lifts the ground connection by using two diodes wired in parallel in opposite directions. This breaks the low level signal loop but allows current through if you have a short on the chassis.

Yes, they really work. You could easily make one yourself if you wanted to. They are kind of expensive for what they are but they work.
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Since no one has said so, I would contact SMc Audio directly.
Pat or Steve (McCormack) can give you the best information. If necessary, they can either repair or upgrade the amp. They can also tell you if the seller sold you a defective amp.