Blackie, if you've already tried the simple(and cheap)stuff like plugging into different outlets, maybe some expense is necessary, but a "power conditioner" shouldn't be the answer. Since removing the input cable to the sub stopped the hum, cutting the ground loop at that point appears to be the answer. An audio frequency ground loop isolator such as this Xitel item installed between the receiver and sub should do the job.

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Enjoy the music, not the equipment.