Subwoofer

@markprice 

glad to hear you mention how adding subs has not only added bottom end but has enhanced all the sound. This has been my results as well. I listen to any music eith guitars and find a well integrated sub bring weight to the guitars and the vocals and the drums. I crossover at a higher rang than most as I like to have the sub help with those midrange frequencies as well. To the op yes two or a full array is always better but one will still sound great when set right. 

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.