Reply to Background 'hum' with 2.6 and Wharfedale 15s.

I recently updated to Traktor 2.6 (ASUS NJ61 running unbloated Windows 7) and have since developed an issue with my personal set-up. I run a pair of (self-powered) Wharfedale 15s directly from the BALANCED output of the S4 (TRS jack to TRS jack) and now notice a faint 'hum' on start up. The noise stays at low level with increased output on both 'sotware' and 'main outputs' - I keep both at around 12 o' clock. I am assuming that it is related to the speakers in some way, as I run a studio set-up with KRK 6s and a 10" KRK subwoofer (balanced output to subwoofer, balanced outputs to monitors) without any issues. Any ideas?
Thank you.