Supplementing KEF Ref 3 METAs

I'm using a pair of KC62's in a room smaller than yours. I switch between R3 Metas and LS50 Metas. The key for me is getting them placed and tuned properly in an L-shaped dining/living room area. They work quite well considering the room conditions. I'll be selling one of them as I'm not a bass freak.

I have the pre meta Ref 3’s. They were upgrades for pre meta R11’s. I found the Ref 3’s to have substantially tighter and lower bass than the larger R11’s. I ran them both full range. 
I have one Rythmik F12SE. With the Ref 3’s, the Rythmik high pass was reduced to 40hz to properly integrate as measured by REW. So it adds some, but substantially reduced value vs with the R11’s. Regardless, I believe the F12SE to be an excellent match for your KEF’s. 

@greatdivide71 Why would you connect the F12s to your KEF terminals?

I am also running a pair Rhythmic F12SE with my KEF R3 metas.  I use a SHD Studio to perform the system's bass integration.  It is like discovering my music collection all over again.

@madfloyd just seeing your question.  That was one of the options/recommendations from the PS Audio folks.  I tried direct to my tube preamp and also off my Hegel amp.  The F12's sounded tighter and better aligned off the KEF terminals using the high level input.  The REW Spectrogram was smoother as well.  I did have to flip the polarity of each sub.  They disappeared (in a good way) after.