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

@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.

@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.  

@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!