Recommend a sub for Kef R11 Meta

Depending on budget, it would seem logical to consider a pair of KEF subs as mentioned above. Doesn't it seem logical the KEF would design their subs to match their speakers?

Don’t get caught up on technology.  Either Rhythmik or REL (and probably KEF) will work very well, but you’ll get much more performance/$ with Rhythmik as they sell direct, and they also offer a 45-day return policy.  I’d highly recommend giving them a call. 

At the risk of sounding like a broken record, here’s another endorsement of the Rythmik subs. Like @thriftyaudio, I have a pair of F12SE (with the GR Research cones), and also use an active crossover (in my case a Sublime Acoustic). 
 

In my case, asymmetric placement of the subs worked best for dealing with room modes. 

Rythmic fan a well. Single 15 in sealed. 

Subwoofers are very room dependent.  Can you tell us about your room and the music that you like to play?