No sound coming from DDJ SX2 to KRK speakers

When connecting the DDJ SX2 I'm not getting any sound output via my KRK speakers. When I change the sound output to built in, I get sound coming through the Mac speakers and they respond to the faders on the mixer. 

All levels and gains are pulsing. Master volume as well, so signal is definitely there. I've upgraded the firmware as well. I've previously used a DDJ RB and it works fine. I'm using a phono to phono cable plugged into Channel 1. Any idea why the sound just won't come out of the speakers??

Thanks