Unexpected Noise 'Source'

Background:
 
Tracking room w outboard hardware, thru Resolv 80As, dead quiet, underfloor unbalanced 6 channel (+HDMI) feed to 
Listening Room.
 
Docking desk for Remote Studio laptop, thru Yamaha HS50s, underfloor unbalanced 6 channel (+HDMI) feed to Listening Room.
 
Listening Room, Yamaha MG124cx or Behringer 1622 mixer, RS 15 band GEQ, Sony preamp, Sony poweramp, Benringer Eurolive 12 stage monitors.
 
All other audio routings balanced,
 
 
AC/Mains setup:
 
Tracking Room on multiple 6 way surge protectors, daisy chained to single duplex outlet on dedicated 20 amp circuit. 
 
Remote and Listening Room each have two daisychained 6 ways, fed by a single 20 amp non-dedicated circuit.
 
 
Problem:
 
The tracking room is quiet, and the Remote used to be. At a point several months ago, with no change in hardware, I began to get low level digital noise in the Remote's Yamahas, partially exacerbated by mouse movement. This was not embedded in recordings made with the system.
 
The Listening Room is internally quiet, with the exception 
of very light air present at higher gain levels thru either mixer. I've determined this air is a product of the Behringer monitor cabinets, by process of elimination, and can live with it for now.  But when I bring up feeds from the Tracking Room or Remote, I get annoying digital noise on the listening room monitors, at below meter levels, but enough to make 'silent' system background untenable, and audible at normal monitoring levels behind tracking signals if listening close.  Working with standard noise abatement procedures, I isolated each device and cable in the Listening Room chain, then both the TR and Remote chains, and came up with...nothing. From the laptop to the Behringers...from the dual PCs in the TR to the Behringers, there is no way to eliminate the digital noise. I was facing the prospect of concluding that all yhree computers had an identical anomaly that did not sum, in phase or not, and further which, was only audible in the listening room, not in the TR, or...wait...  This first appeared on the Remote, pull the only piece of gear not yet tested, the powered Yamaha HS50s, and when I did, the whole system went quiet.  This is somewhat surprising, since the M-Audio C600 interface on that DAW has pushbutton A/B/C monitor switches, and the noise is present whether the monitors are live or on standby.  In writing this, I've realized I have one more set of tests to run, checking the HS 50 power and TRS cables and both left and right contributions. I'm on a tablet which had (note past tense...thx bakers!) forum issues, and I don't want to lose this, so hang tight, BRB...
 
 
 All cables good.  Noise includes 60 cycle hum and higher order harmonics, in and out digital noise, 50 % of the time, period one half to one second.
 
W Yamahas connected, noise present whether the m internal onitor amps are switched on or off. Noise goes away if monitors unplugged at interface, or at 2nd 6 way  surge protector, or if laptop power supply unplugged ar either end.
 
If interface power supply unplugged  either end, (leaving it USB powered) noise in Listen Room disappears, but remains  in Yamahas.
 
This tells me there's a ground loop involving, most likely, the interface and laptop, but possibly including the monitors.
 
Suggestions?