Gog, welcome. The fact that the hum existed when plugged into outlets in different rooms raises at least a possibility that this isn't a ground loop hum. If you disconnect the sub cable(while it's humming)and just have the power cord as the only connection, does the hum still exist? If so, there's no connection to form a path for a ground loop, and the problem is in the sub amp. If not, the next thing to try would be removing the chassis ground screw on the back of the sub amp.

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Enjoy the music, not the equipment.