Supplementing KEF Ref 3 METAs

What are you trying to acheive? 2 subs to fill in room nodes for even bass around the room or do you want the best possible quality bass sound?  

This post he wants to run 2 subs in stereo. Please read.

https://forum.audiogon.com/discussions/rel-classic-98

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?