Oh sorry I feel like I should have to have them in the level in the plus range in the avr with the gain controls max and sensitivity at .775 to get them moving even at -30 volume on the avr. Each channel is rated at 1350w on xli so I'm wondering if it's a preout voltage issue or something.
The RCAs are not causing the buzz in your system, and XLRs will be no better in that regard.
The first thing to understand is that a ground loop by definition is a potential between grounds, so that a current will flow between the grounds. So changing cables and outlets is not a solution.
What you have to do is find the ground, or grounds, that are not at system ground.
A very frequent cause of ground loops are cable systems and Ethernet connections.
So this is how you go about isolating a ground loop.
First you just plug in the power amp. Now with everything disconnected from the receiver or preamp except the power cable, connect it to the power amp with your RCA cables. Is there hum? If not then the receiver/preamp are at the same ground potential as the power amp. If there is hum then there is a faulty grounding in either one of those units.
If no hum, then connect the cable system to the rig. If you have hum then the ground loop is coming from the cable system. If there is hum disconnect the cable system. If no hum, then leave it connected. Now connect the Ethernet cable. If no hum leave it, if hum disconnect it.
Now do this for every unit that is connected to the system. This way you will identify which unit or units are causing the ground loop.
Once you have completed and identified all ground loop sources report back and I will suggest potential solutions.