Question about hum from speakers

@bhakti-rider
Here ya go!
https://www.hometheaterforum.com/community/threads/cheater-plug-really-dangerous.75419/

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.  

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.