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

                                      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!

@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!

     "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!

@millercarbon - right, but if you want to get your hum levels down to below 70 -75 dB, you will need to do more than just set wires a few inches apart.  Your comment is not informed, it is just biased, and incomplete.

@rodman99999 - Apparently, the original posting link and paper was deleted for some reason.  Unfortunately, MS Edge didn't think the link was important, so it dumped it along with a number of other informative links.  

Regarding the spacing and 90 degree crossing, all I can say is, when I put a voltmeter on my system and fought to get hum and hum related noise below -70 dB, it took an entire 3 day weekend.  I found that I needed dual ferrites on the less expensive wall warts that were several feet away from the phone inputs and my big Marantz Class D amp had to be two horizontal feet away from the Stax Headphone amplifier.  I discovered any low level hum at all dissolves clarity and definition, even at louder volumes, so the long effort was well worth it.  

     Was there something to read, in the original post? Didn’t show up, here.

     At any rate: if one considers the low voltages/currents we’re dealing with, as regards the typical system’s interconnects; the Inverse Square Law works nicely, with a few inches distance, when faced with parallel runs.

     AC cables and interconnects (particularly so, if unbalanced) may prove another story; especially, if the interconnect feeds a higher gain circuit (ie: phono).

     Crossing cables of any kind, at 90 degrees, is sometimes the only option and will typically avoid induced currents.

               http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/isq.html

     https://www.thepodcasthost.com/equipment/cable-crossing-bad-thing/#

Not really. I scanned enough to realize nothing new. I also realize there's people do nothing but post glib uninformed comments. Mine is glib, but informed. I know there is more to it. But if you want to help people without taking up a whole afternoon boring them to tears with techno jargon, route wires a few inches apart and at right angles when space isn't possible.

@millercarbon -  if you want to scrape  the last vestige of hum out of your system, you will need to do a LOT more than "keep them all a few inches apart."   You do need to read the paper if you think that is all there is to it.

Or you could just keep them all a few inches apart, and at 90 when this isn't possible. 

There. Now THAT was a pretty easy read!😂