We returned the P300HD sub. In addition to the hum, it was much too heavy for us to maneuver from place to place. We will replace it with a CSW P205.
To avoid a ground loop, we will first try connecting the P205 with an RCA plug to the receiver's subwoofer out. If the hum appears we'll use the alternative connection via RCA stereo signal plugs to the receiver's L and R preamp outputs. If hum persists, we also have the third option of connecting the sub with speaker level connections.
As to our hum-free P1000, Anthony DiMarco wrote the following:
"...Forgoing a potentially expensive, typically hum-sensitive RCA-type unbalanced interconnect for an inexpensive two-line RJ11 phone cord, Newton engineers were able to create a fully balanced, hum-proof connection between the sub and controller module. If anything was proven during the course of my evaluation, it was that this single feature worked like a charm. No matter how far the sub was from the controller, no hum ever ruined my fun" (Home Theater Sound, August 2001).
Perhaps I should rephrase my original question: Beside the CSW P1000 and P1005, are there other subs that use a fully balanced, hum-proof connection?
I've attached some illustrations to show how the P1000 is connected using RJ11 phone cord. Although the attached illustrations show connections for the CSW P500, the connections are identical to those of the CSW P1000.