Sub to complement KEF LS50s: SVS vs. Rythmik

     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).

You could try two Gallo Acoustics TR-3D subwoofers like I do with the LS50s (I actually have the older versions). Also live in a NYC apartment about the same size, and they fill the room for music and movies. They have a small footprint and aesthetics that don’t look bad with the speakers. I haven’t tried room correction just setting them to crossover at 70-50 hz. Have the two shooting from the front, but I think it sounds best with one in the front and one in the back. Hope this helps.

has anybody tried small.. like 8 inch or 6 inch subwoofers? Is their such a thing with proper extension and frequency response?

I’m currently using LS50 and two SVS SB-16 Ultra and I have been SUPER impressed with the SVS subs.  The only down side I can mention is size and weight.  Besides that, they integrate perfectly with the LS50 speakers.  The sound stage is wide and open with incredible 3D imaging.  Of course, most of the time with music the subs are invisible completely and barely moving.  I’m using 70hz crossover.  Most importantly, I recommend two subs versus one for all reasons mentioned above.  Good luck!