Y adapter beneficial for sub cable?

HI Asa- it depends on the sub you p[lan on using.  Personally, I would use a REL sub which is easy to integrate into the system and runs off the amplifier terminal.  With a low level (RCA/XLR input) sub, if it is a stereo input sub, you are in great shape with a standard pair of interconnects.  If it is a single input sub, you MIGHT be able to use a "y" connector on the preamp outputs to combine channels and you might not.  With some designs this will result in a mono signal for the sub (which is fine) as well as a mono signal for the LS50's ( which is NOT fine).  Of course, if you have a LFE output on the pre, that would work fine to run the sub on a single cable.  Good Luck! 

Thanks for the reply,

The KEFs only have one sub output from the internal amp. The sub I am considering is a SVS SB12 NSD. It has both L/R inputs. This is for my man cave/delesktop setup and the room isn’t that large so I’m sure the SB12 will be more than enough, if not too much. My needs are 100% music.