Mixing front and center

I would focus on matching tone/timbre, and since your speakers have a silk dome tweeter I’d try to stay with something similar and for that reason would shy away from KEFs with their metal tweeters.  This Aperion gets solid reviews from users and in reviews, they offer a 30-day in-home trial period, which is really nice, and it’s over $200 cheaper than the KEF.  Importantly it has a dedicated midrange driver mounted below the tweeter that is superior to the more common MTM design of most centers in this price range (concentric designs as in the KEF are also good).  Hope this helps, and best of luck.

https://www.aperionaudio.com/products/verus-v6c-center-speaker

Well, first, do you need a center?  If you sit in 1 location all the time, the center is not important.  Yes, I’ll get flack for saying this but I have a lot of professional and personal experience.  The center really shines when you are moving off center.  It keeps the dialogue and action placed where it should be as you move around the room.  

And then yes, matching the timber of your current speakers helps a great deal, but  you can somewhat mitigate this with excellent room correction. 

Above all, avoid 2-way, center speakers.  They have limited off-axis response.   This doesn't apply to concentrics.  You could use a single Dynaudio speaker as well, if you can place it normally.  A normal bookshelf 2-way speakers on their side have lopsided dispersion. 

Erik S. as usual is spot on. The Dynaudio Contour 25C uses a 2.5 way crossover, with the bass/midranges in parallel below 300Hz, but one of them crossed to the soft dome at 2.5kHz. This technique avoids the worst phase cancellation effects so commonly associated with 2-way centers. The KEF looks like more speaker for the money, but the metal drivers worry me as to timbre match with your Dynaudios. Why not buy both and compare?

@dynacohum  - Oh that's a neat application of a 2.5 way crossover! 

Get another pair of Focus 140s. Use one as a center and the other as a spare.