Recommend a sub for Kef R11 Meta

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

+1 Rythmik

Paired Rythmik F12SE with my R11’s. Great pairing, very musical. Great SQL. 

Agree with the comments of 2 subs to match your 2 towers.  I’ve had a great experience with SVS and RSL subs.  My favorites for music in a tight space are SVS SB 1000 Pros.  I also have a SB 3000 and SVS will be releasing a new 3000 sub line. I prefer sealed subs over ported for music listening.  

There is no servo control in REL subs. As a result, Rythmik subs are generally reported to have lower, frequency-dependent group delays (GD) than REL subs. I wish I had measurements to show you. On the other hand, REL subs are still often rated as more musical, mainly because they tap the speaker-level signal and can integrate more seamlessly with the main speakers, introducing much less perceptible phase shift.  

As a comparison, SVS subs tend to have relatively higher GD than the nominal 1/f. I have tried two of them (SB‑1000 and SB‑2000) and found that they are not quite as fast or as precise.