Sub to complement KEF LS50s: SVS vs. Rythmik

jdtalv, the SVS, and I've never even seen one. 

  Your going to need all the signal processing you can muster to begin a worthy crossover between the two unless you've considered an even more sophisticated method of manual multi band parametric variable phase, gain, and Q DSP, in which case most any box will do.

  Read as many DSP setup procedures as possible for a better understanding of how using a higher crossover point and adjusting the Q can attain a smoother transition that will more closely match your main speakers presentation in the crossover region.   

  My experience with integrating powerful subwoofers with small speakers has been quick and easy using a sub that seems to be out of production because of the manufacturers success in another field of endeavor. This same manufacture patented servo monitoring and control and found a better solution three generations ago. (calvint, you gave up too soon)

  This is in no way a substitute for proper positioning or the use of multiple subs which are delightfully explained here, elsewhere. Have fun with it.  

M-db

I owned 6 different subs from that manufacture, starting with their original 80s design. My  Rythmiks represent a step forward.  

mcreyn, 

As I said, I've never even seen one.  

Nice to hear of your satisfaction though.

My 2 cents from the 4 years of listening and enjoying experience with this magnificent speakers.

Buy a pair of competent subs that can go clean and flat as high as 100-120 Hz, because LS50’s response starts to weaken at these frequencues especially if you have them far from the back walls. If possible, cross them over at 100-120 to further relax them and improve the mids. Keep the subs close to the ls50 and you will enjoy. 
You can leave them full range as well just dial the sub volume adequatelly.
I enjoy them with a single b&w asw608 sub and the bass is adequate at the listwning position and very clean and articulate.