Reply to Connecting powered sub with S4

I run two krk rocket 5's as my main monitors, plus a truth/behringer 2092a dual 8" powered subwoofer.

Just got a s4 and I'm having a hard time connecting the sub.

The krk's are hooked up to the mains(two separate rca's but nothing to connect the sub?(I have a xlr to RCA cable).

I usually connected my sub to the "booth" input on my old controller, nothing on the s4 seems to work...
Thoughts?