Supplementing KEF Ref 3 METAs

TWO of the KUBE 10/12 MIE would be a far far better option.  Unless you desperately need a micro sub, you're giving up SO much performance to save space.  

We sell a lot of R3 Meta + subs systems.  It can almost sound end game when set up right (ideally bass managed, and time corrected).

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.