In your owner's manual page 18 & 19 it describes the use of rear conntections. If you have a stereo preamp without an LFE OUT then use either both RCA (unbalanced) OR both balanced inputs. The unbalanced RCA inputs should not hum because internally they are optically connected in the sub. In my experience (F113 v2) the source of low level hum is further up the chain. I would unplug all equipment connected to the pre amp to see if the hum goes away,...and if it does add one component at a time until you find the source.