Gog, apparently the Axiom site info is outdated or simply incorrect as to the recent EP350s. It continues to both illustrate and suggest the use of the "Chassis Ground" screw in cases of possible ground loop hum.

Using a "cheater plug" will certainly stop the hum, but the absence of the previous chassis ground screw raises a question as to whether there'd be an alternate ground through the sub cable or if there'd be none at all if the ground through the sub power cord is defeated. Axiom should clarify this.

Yes, disconnect the satellite cable to see if that's the link in the ground loop. The Axiom ground loop unit isn't suitable for satellite TV use since it would interfere with signals to the dish.
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Enjoy the music, not the equipment.