Mongo, a puzzling point here is that two of your observations appear to be directly contradictory: in 2)when you used the cheater plug, it should have broken any ground loop, and the fact that it didn't stop the hum indicates that a ground loop isn't the problem; in 5) when you disconnected the input cable from the sub, the hum stopped, which indicates that there was a ground loop which was broken by disconnecting the cable.
Since disconnecting the cable TV lead(a common ground loop source)didn't help, a ground loop isolator for the cable TV shouldn't help. You might try an audio frequency ground loop isolator such as this
Xitel item between the receiver and the sub. However, the fact that the cheater plug didn't work, as mentioned above, raises a question as to whether this would be helpful.
