I was curious to see what this was, and fetched a couple images off the internet for visual aids:
When you have this opened up, does anything look like it's missing, or cable routing suspicious? Any hardware missing? Such as shielding that might have been in place to shield the RFI these switching supplies will generate? I've never been inside this model before. Also, somewhat odd that using the AFL monitoring, all the input channels don't exhibit the hum, while the buss outputs do. Now, buss amps can pickup stray fields and have lots of gain. Cable routing on the power supply from the SMPS to the system may be critical. There is lethal voltage on that power supply, so make sure it's not connected to the AC mains when trying to vary the lead dress in searching for the source of hum injection.