Sub to complement KEF LS50s: SVS vs. Rythmik

Yet another option is the Vandersteen 2Wq sub.  Very musical sub with a unique integration system.  Here's one available now...

https://tmraudio.com/speakers/subwoofers/vandersteen-2wq-powered-subwoofer-wx-2-crossover-walnut-fin...

I buy buy into the multiple sub thing too, but if you go with one sub you can do far worse than this Vandy for the money.  Best of luck. 

I have been using 2 SVS SB-1000s to give a bit more kick to a wonderful pair of PSB T2s for years.  My NAD 390DD DAC/amp allows for a flexible crossover point and low pass gain adjustment.  Adding Dirac Live to the mix helps even things out and lock timing down.

I would add that smaller subs are just less ridiculous looking in a room as well.  My SB-1000s (white finish with white grilles) both sit on the bottom shelf of some pretty standard maple bookcases.  No one even notices they are there.  The bigger subs you mentioned above are certainly capable - but there is no hiding something of that size.

But - if I had a more dedicated listening space where aesthetics were less of a thing - the Swarm system mentioned would be high on my list.  I have read about it several times over the years and am dying to give it a try.

     There's definitely an abundance of very good quality subs available, both new and used, that will produce good quality bass in any individual's room and system at a single designated listening position.  I suggest we all agree on this point and dispense with the typical forum phase of having a series of posts from various individuals that recommend the OP use the exact same brand and model sub that they purchased and use in their systems.
      If the goal is to provide useful information and advice to the OP to help resolve their audio concern or question, however, it's probably best we just skip ahead to the useful information and advice giving bits.  In this spirit, if the OP desires better than just good quality bass performance and integration with their main speakers at their listening seat, a very good solution is to utilize two of whatever subs they choose rather than a single sub.  
     The truth is that the number of properly positioned subs deployed in a given room and system is more important than the quality of the subs used in determining the overall bass performance quality perceived at the designated listening seat, even though utilizing higher quality subs will also improve the overall bass performance quality. 
      In general, two subs will perform twice as well as a single sub, four subs will perform twice as well as two and three subs will perform significantly better than two but not as well as four.  The ultimate bass system, in terms of bass detail, smoothness, impact, dynamics and overall quality, that I'm aware of is definitely the 4-sub DBA system, whether it's the complete 4-sub Swarm kit or a custom 4-sub DBA using even higher quality self-amplified subs selected by the user.

….."I suggest we all agree on this point and dispense with the typical forum phase of having a series of posts from various individuals that recommend the OP use the exact same brand and model sub that they purchased and use in their systems."....

But this is what the OP is asking for, which of the two subs he is interested in is the best, preferably from someone who has heard both.  It is pretty intuitive that the more subs you use the fuller your bass will be.  But as mentioned above, if your set up is in your family or living room having multiple subs may not be possible (although I have two).