Found the schematic.
https://el34world.com/charts/Schematics/Files/Mesa_boogie/Boogie_dual_caliber_dc5.pdf
Only difference between jumpering with a cable and just having nothing connected, is that you put the mix pot in the circuit. So it’s perplexing that having anything plugged into the loop causes noise, versus having nothing connected. I am also wondering if the 1uf electrolytic cap right before the loop is bad.
The key to try to figure out is if the noise is coming up stream from the loop or downstream. Taking the send jacks into another amp and comparing this to plugging a guitar into the return jack is a good place to start. When you plug into the return jack, you want to put a cable in the send jack, so that the preamp of the dc5 is disconnected, also make sure you have the loop level maxed in this test.
Also if the graphic Eq effect the hum, that would suggest the noise is coming in before the loop.