Try it with the RCA. If you experience ground loop noise the XLR out would cure it, but you would need a balanced to unbalanced converter at the sub end to use it. An XLR to RCA cable does not convert balanced to unbalanced, it just makes the entire connection unbalanced and won't stop ground loop noise.
Bill Fitzmaurice Loudspeaker Design
The Laws of Physics aren't swayed by opinion.