So right now you only have power cords, HDMI and XLR (and speaker wires) in the system correct?

Have you tried unbalanced connects from the Marantz to the Amp? (Takes the XLR out of the equation)

Have you tried running component to the TV from the Marantz, taking the HDMI out of the equation?

If either of these works then the grounding scheme related to the cable removed is wrong/broken.

I have some other suggestions that may provide insight however they may cause damage to your speakers, so we'll leave that on the table for now. Basically I want to get the Marantz out of the loop and feed audio directly from the TV to the amp but the TV specs don't show analog outputs so I don't know how to do this gracefully. Any ideas?

Do you have a volt meter?