I added the line on USB after you grabbed the quote. I swapped the Ethernet for some cheap cable I had, but I just found out the AQ Pearl I tried is shielded. I have another issue of intermittent humming in the Alexia midrange, but may from the high A/C activity this time of year. I've been working with very kind post-production engineer from a download site; he has a few techiques for eliminating the ticks from downloads since often there are ticks in the material he receives. It may be a matter of how senstive a given system is to the DC offset, as well as the amount of DC offset, and the type of music (noticeable in quieter music that segues tracks).
Have you tried plugging in a AC tester. Has 3 lights and tells you if you have hot, neutral and ground right on your 3 dedicate lines. These gizmo's are like $5 at home depot. Just a total shot in the dark on this...
https://www.homedepot.com/p/Power-Gear-3-Wire-Receptacle-Tester-50542/206212329