Supplementing KEF Ref 3 METAs

I have done exactly this in a room about the same size, but with a high, sloped ceiling. The bass integration is quite good, but the 62s really are too small for the space. I’d move up a notch or two for your space. 

I am using two Rythmik subwoofers with my KEFs in a 16x24 room.  Very pleased and integrates well.

budget?

I picked up a local kc62 last week off marketplace. I have it set at 65-70hz sitting in the corner with the corner setting turned on in the dsp. Running CSS 2TDX and a Simaudio I-5. So the speakers are running full range. Tight punchy bass that actually sounds like it’s an extension of the mains. Very impressed with this little guy. Room is around 15ft x 11ft and it’s plenty. It’s better than the Velodyne Optimum 10 that just developed a static issue recently. 
 

I have a JL Fathom F113 in another system in a bigger room. While the KEF has no where near the output of the Fathom it’s tighter on the upper bass. 

My room is treated 14'x20' -- I have KEF R3, high passed at 80Hz. Below 80Hz I have a KEF KC62 AND a KEF KC92, filling out the bass. I love my KC62 microsub and it is a really impressive little thing that goes down to 11Hz, but if you want power and full room coverage you'll need some more power. That's why (after living with a single KC62 for a while) I added the KC92, which is equally impressive and although it doesn't necessarily go deeper, it does go louder. KC92 is obviously a bigger box than KC62 but still visually appealing, and not excessively large. If you only play at low volume, intend to play the R3's at full range, 1x KC62 may just be enough to fill in the low-end. But if you pay loud, and/or watch movies, I'd recommend 2x KC62's, or even 2x KC92's. Also the suggestion above to get 2x KUBE 10/12 MIE's is an interesting route to explore. Good luck, and I hope you don't have fussy neighbors!!! :-)