Sub to complement KEF LS50s: SVS vs. Rythmik

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!

I think REL makes an 8 inch sub Ti/5?

I use this sub with my ls50s.   8" active driver and 2 8" passive design.   matches well to the ls50s and delivers a lot of bang out of a smaller package than most.  Highly recommended especially if space is a consideration.

https://www.klipsch.com/products/sw-308-subwoofer

jetter:"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)."

Hello jetter,
     Yes, the OP was asking opinions on two subs from those that have heard both.  He also stated he was just "looking for a top-notch single sub and just be done with it". 
     I decided to point out that it's not that simple and that a single sub in any given room will not deliver the level of bass performance some might expect but that two subs , properly positioned and configured, are capable of providing bass performance at a single dedicated listening seat that is roughly twice the quality level of what a single sub is capable of providing.

    It may be intuitive that the more subs you use the fuller the bass will be.  However, it's much less obvious and commonly understood that the more subs you use, the more detailed, faster, smoother, more natural and realistic the bass is perceived due to the effects of psychoacoustics. This is in addition to having increased bass output, impact and dynamics with each sub operating stress free and well within its limits.  I considered these facts as important for the OP and readers to be aware of when determining how to achieve better bass response in their systems and rooms.   I'm not sure what your point is, do you disagree with my view that these added psychoacoustic benefits derived from using multiple subs are important ?
     I do agree with you that accommodating multiple subs in a living or family room is a common and legitimate concern.  I had this same concern when I decided to purchase the 4-sub Audio Kinesis Swarm DBA system.  I use my 23' x 16' x 8' small to medium sized living room for both music and HT purposes.  I previously had a lot of experience using and accommodating fairly large single and dual subs in this room with minimal major room furniture rearrangement but also without significant bass performance improvements.
     I had done a lot of research on the 4-sub DBA concept and had very high expectations of significant bass performance improvements.  I think the prospect of finally attaining near state of the art bass performance in my room that was well integrated with my fast Magnepan 2.7QR speakers, that I had been  trying to achieve for about the previous decade, may have influenced my decision to commit to just positioning each of the four subs in my room where they performed optimally and dealing with any required rearrangement of room furnishings afterward as best as my wife and I could.  Luckily, I have an understanding wife who also enjoys high quality audio reproduction on music and HT.
      The Swarm subs are heavy at about 44 pounds/each but relatively small at 12" x 12" x 28"/each.  It also helps that the front firing 10" drivers are designed to be positioned facing and just a few inches away from the nearest wall, with all connections hidden on the bottom of each sub. 
     The optimum positioning of the subs in my room turned out to be two along the front 16' wall (with each about a foot away from the left and right corners) and one along each left and right 23' wall (with each about a foot away from each rear corner).  The only furniture rearrangement required was moving an 8' couch along the right 23' wall forward by 2 feet.  The front two subs aren't even visible from my listening seat near the rear 16' wall since they are each hidden almost directly behind each of my 6' x 2' panel speakers.  If you're having difficulty imagining my description, I have pictures posted on my profile page.
     My main point is that the Swarm's four relatively small subs blended within the room very well and better than my wife and I and probably most people would expect.  The bass performance in my system is now what I honestly consider state of the art on both music and HT.  The bass is whatever the content calls for, fast, quick and taut or deep, sudden, powerful and impactful that you feel as well as hear. 
      The bass is seamlessly integrated with the main speakers in their similar qualities of being very quick, agile, detailed and always smooth and natural but now with the added dimension of the bass being powerful, impactful, dynamic and fully extended down to a flat response on deep frequencies to 20 Hz.  
     My point is that I was more than willing, if it resulted in attaining near state of the art bass performance in my room and system for music and HT, to rearrange my living room furnishings to gain these benefits.  The fact that I only needed to move my couch a few feet was not only fortunate in my case, I think it's a lesson learned that may also apply in other individuals' rooms.  I believe that, ultimately, it's a personal decision as to what compromises individuals, along with their significant others,  are willing to make in their rooms for the sake of attaining near sota bass response performance.  I would just suggest that the relatively small and attractive subs utilized in the Swarm 4-sub DBA system are more likely to blend in as seamlessly visually into one's room as they do sonically with anyone's main speakers. l  think it may be more difficult if a custom 4-sub DBA is created using larger subs.

Tim