Subwoofer

Bass play a great role enjoying the music...Even for localization ( phase angle) and imaging and soundstage...

And hearing tuba clearly and deeply and organ bass note is like , time to time making love with a plump woman and not always with an anorexic, an another cosmos of love or sounds... :)

One sub only , everybody say it, is very difficult to balance ( impiossible for me in my experience)...

You are spiritually and syntactically irreplaceable here... I am glad to spoke to  you again...

@mahgister , it is good to 'see' you again...*vbs*

As one who's lived through both of the '60's', we're becoming rare instead of being merely 'well done'....;)

As for 'subbing it', a single self driven 8" for the moment, considering an array of 4; 2 - 8~10" left & right 'front", 2 - 6~8" L & R 'rear'...although 'placement' and levels subject to measure and taste....self-driven, perhaps...

All of which subject to the vagrancies of time, space, and budget....I need obedient clones....*L* 

+1 on calling SVS ... they can help 

And great to see @mahgister ​​​​and @fuzztone in same discussion two of my favs!! 

Given your limitations and speakers it might make sense to try the KEF offerings as they have settings to support non-optimal placement. The KC 62 might be a bit small for that room, the KF92 might be a better choice. 
 

I have LS50’s, and one sub is better than no sub. Yes, two might be better, but not required to improve your sound. 
 

One sub is very hard to put in a musical synergy with the speakers...

But if someone want only boom boom sound , it is very easy to plug it...

I plug mine few minutes and sold it... 😁 because i could not think how i would make it "musically integrated" ... i think that it must takes 2 ...

A balance sound is not a boom boom sound added...

But my short experience with ONE sub  ( Kreisel K9 ) was not happy and i could not figure out a way to integrate it "musically"  ...

But once this is said i am not a sub woofer specialist at all...But i love music and i hate boom boom... Perhaps i am wrong and there is a way in some room to do it musically with ONE sub... We must ask to a sub-woofer specialist for confirmation or debunking ...My intuition is location and room dimensions are very important for this problem then there is no absolute rule and in some case one sub can do the job perhaps... In most general case i think it takes more than one...