Hal suggested safety grounding it, but that won't work. Grounding it to the RCA's would. Because all the grounds are connected on the jacks, I'd just remove the inside insulating ring. With a plastic box it won't do too much, but with a metal box it'll prevent incoming RF from radiating so much.
As practice I recommend doing the output jacks, which doesn't always matter but because the inductance is higher after the caps, I'd do it on those two. Then I'd stat ground to them from in and sub-out jacks.