In my experience hum is not normal, if it instantly goes away when you pull the cable it “could” be a ground loop? Make sure you have a proper ground from mains plug? Also might try a better/different cable (in case the one you have is defective)?
I would try the wireless and see what happens (see if the hum is gone after unplugging LFE).
As far as placement, have you done the subwoofer crawl? Place the sub in your primary listening position, play some bass heavy tracks, crawl around the room with your head at the height the sub will be and find the “sweets” spots (where ever the bass sounds the best). That is the spot in the room that is "best’ placement for the sub for that listening position. There may be more than one spot with varying degrees of bass.
I would definitely recommend playing around with the settings to get the crossover etc optimized.
look here for a discussion:
I would look at KEF site to see if there is an updated firmware for your sub and speakers.