FYI, the Marantz AV7702 is already lifted from ground, so you don't need any cheater plugs. You can tell my looking at the IEC inlet on the back and seeing that there is NO ground prong.
You could try this: disconnect all source devices from the Marantz (such as bluray player, satellite receiver, DVR, etc.). Then see if there is ground loop humm at this point. You can connect each source device (probably by an HDMI cable) one at a time to see if there is a source device that is creating ground loop humm.
Another thing that can create ground loop hum is devices that are plugged into a different opposing-polarity circuit. If you have identified a bad source device on the Marantz, try plugging it into the same outlet / power conditioner as the amplifers (same with the actual Marantz).
Sometimes ground loop can come from a cable box or a satellite receiver with improperly grounded dish/COAX wiring. You may need to get a COAX isolation transformer - something like this:
https://www.amazon.com/Ground-Loop-Isolator-Transformer-Eliminator/dp/B08FRGH4X2
They are very easy to install inline with the coax cable that screws into the cable box or satellite receiver. Be aware that these devices may not work on some cable boxes or satellite receivers. Some receivers/boxes can't communicate with one of these isolation transofers on the line. However, sometimes this can improve the image quality and the audio quality of the cable/satellite.
You could try this: disconnect all source devices from the Marantz (such as bluray player, satellite receiver, DVR, etc.). Then see if there is ground loop humm at this point. You can connect each source device (probably by an HDMI cable) one at a time to see if there is a source device that is creating ground loop humm.
Another thing that can create ground loop hum is devices that are plugged into a different opposing-polarity circuit. If you have identified a bad source device on the Marantz, try plugging it into the same outlet / power conditioner as the amplifers (same with the actual Marantz).
Sometimes ground loop can come from a cable box or a satellite receiver with improperly grounded dish/COAX wiring. You may need to get a COAX isolation transformer - something like this:
https://www.amazon.com/Ground-Loop-Isolator-Transformer-Eliminator/dp/B08FRGH4X2
They are very easy to install inline with the coax cable that screws into the cable box or satellite receiver. Be aware that these devices may not work on some cable boxes or satellite receivers. Some receivers/boxes can't communicate with one of these isolation transofers on the line. However, sometimes this can improve the image quality and the audio quality of the cable/satellite.