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?
Disconnect all inputs. If it is still occurring chances are you have a failing power supply component. Not rocket science to identify and replace

If it does not, you have a ground loop. That's very system dependent.
Try running a wire( any kind) from amp to the screw of the wall plug faceplate
Either way you got one of the finest power amps on the market regardless of price. I know, I had one. Very hard to beat. Worth the work if it even needs it.
"I have to believe that the seller was well aware of the hum. "

If that’s the case you should return it and get your money back! Why would you keep a defective amp?
Your power supply cap(s) are shot. Easy to replace if you’ve any skill with a screwdriver and a soldering iron.Likely you had one bad cap, and nobody noticed. When the second one failed, that’s when the AC hum started. And in defense of the seller, these literally can fail overnight.I doubt if it's a ground-loop all of a sudden. That hum would have occurred  with your original amp.

Yogiboy:  I've requested a return for full refund; I'm thinking in terms of what if I can't get that refund.
Thanks for the responses so far.  So it could be a ground loop even though there's no hum from the Dynaco, all else being equal?
You should try a cheater plug and see if that works!
Erik & Yogiboy:  I did disconnect the inputs, and there was no hum.  So I found a spare cheater plug and inserted that, connected the inputs again, and the hum was gone.  Thanks!
Do not use the cheater plug as a long term solution. It does indicate you have a ground loop however.

It also absolves the previous owner. :)
Erik, what is the long-term solution?  I'm using one for my PrimaLuna preamp too, and if there's a correct approach, then I need to employ that for both of these.

That is a safety ground. Lifting the ground could lead to fires and lethal voltages at the case. That's why the ground wire in your outlet is required to be the same gauge as your power line (12 or 14 gauge) : So in the event of a short to the case a breaker will trip.

The best way to fix it is to identify the root cause. Often this means making sure multiple equipment is on the same strip or outlets or adding isolators to any external coax lines (for cable/internet or outside antennas).

Unfortunately it's no one solution. 

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My ground loop was caused because I have my stereo also hooked up to the tv cable box for watching some tv shows.  Buying the following isolator completely eliminated the hum.
Jensen VRD-1FF IsoMax Digital RF Isolator
Try unplugging components until you isolate the component causing the hum.  Also note, that I have had RCAs on my turntable that were not insulated enough and these caused hum, which when replaced, went away.
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.