An excellent paper on routing cables to avoid interaction and noise injection.

     "I discovered any low level hum at all dissolves clarity and definition, even at louder volumes*, so the long effort was well worth it."

                                           *I couldn’t agree more.

     If I'm not mistaken: the better Stax headphones/amps work at much higher voltage levels than most, which could (possibly) account for the distances necessary, from your other components.

                   KUDOS, for persevering and lowering your noise floor!

@rodman99999 I believe you are right.  I seem to remember it had a 100Vrms output or something like that when I bought the unit.   The Stax chassis is aluminum, so magnetic lines will pass right through it.  I thought about covering the inside of the chassis with conductive copper tape to make a shorting ring but since moving the Marantz fixed the problem, I'll leave it as is.

Thanks for the comments!

                                      You're welcome.

       Regarding the Stax pieces: I was thinking about the bias and signal voltages, that some of their amps supply to the electrostatic phones (little current/big voltages, relatively speaking).

                          ie: https://staxaudio.com/technology

                                       Happy listening!