50ASX2BTL Balanced Ground Loop

I'm trying to wrap up my 11 channel 50ASX2BTL home theater amp, and I got a terrible ground hum during test and incorrectly grounded it somewhere. OEM documentation is weak and searching has not produced any relevant results. You can see that things are a bit tight, and before I start taking anything apart I wanted to ask for some help.

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The chassis is grounded to mains earth, and pin 1 from each XLR is also connected to the chassis. The PCBs are connected to the chassis using aluminum bolts, and which has grounded each PCB to the chassis via two metallized contact pads on the PCB at the bolt holes. This is one thing that I figure might be the cause, although I thought that this was the correct way to tie together the grounds.

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For ease of construction I have used the Ghent pre-fab cables, but they are not shielded and did not include a wire to connect to the signal ground pin on the JST connector. I made my own XLR patch cables using mogami shielded cable, and I connected pin 1 at each end of the cable, and as stated above Pin 1 on the XLR. The ground hum shows up even without any signal cables connected so presumably my patch cables aren't the issue.

For anyone that might ask, I plan to add my MiniDSP 2x4 into the case along with a 12V supply to provide auxiliary power for several small items. I will add these later, as well as wiring a 12V on/off remote trigger.

Any help is appreciated. Thanks!
 
Try using thick low resistance copper wire to join things together, eventually the loop should disappear... Often on many PCBs (instrumentation etc) you will see a spare hole for adding a low resistance cable connection to troubleshoot or bypass a ground loop problem.
Using aluminium bolts and the PCB pad to connect to ground is not the best way, without correct torqueing etc. the connection can be iffy, try using cable, the best bet if required is a faston connector on the PCB for a wire connection to chassis.
I cant see the pictures at the moment so can only give generic comments.