Recommend a sub for Kef R11 Meta

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?  

If you have the space, I would also recommend 2 subs.  I have 2 JL Audio e-110’s, and they blend great with my Raidho XT-2’s.  I did buy them at least 5 years ago and the price is now up to $2,200 each.  I think that they were around $1,700 each when I bought them.  They got a fantastic review by Jonathan Valin of The Absolute Sound way back in 2014.

https://www.theabsolutesound.com/articles/jl-audio-e-sub-e110-subwoofer/

I am looking at the same REL HT-1205 for my home theater.  IMO it would be a great choice as well.

I have been warning folks on Audiogon about spending too much on powered speakers/subs.  Expect that any powered speaker/sub will likely last about a decade or perhaps longer if you don’t use them that much. If they fail, Class D amps are notoriously hard to repair and costly to replace.  Keep that in mind.

SVS SB-2000 Pro would be my choice to try since they offer a trial period.