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?
bhakti-rider
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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

https://www.parts-express.com/jensen-iso-max-vrd-1ff-75-ohm-inline-digital-catv-isolator-hum-elimina...
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.