I'd avoid the 1/4" -> RCA adapters and ensure you're using TRS cables -> whatever your monitors support (XLR or TRS). This will help.
This is a decent starting point to run through in order. https://support.native-instruments.com/hc/en-us/articles/210293145-How-to-Resolve-Ground-Loop-Issues-Humming-and-Buzzing-Sounds-
What I'd do next:
-ensure you're using balanced cables and inputs on everything
-try the ground lift xlr adapters
-try DI box.
If you have a mate with the adapters and/or DI box to test first that's always a good way to go.