Very small room systems and synergy

Lets not be hasty here. It may be that what your are hearing has little to do with Metal Dome Tweeters.
It's likely that the overall sound is characteristic of combined effects.
Amplifier / Interconnects / Position.
"  What I find is a lack of overall frequency response "
This sounds like you are right on track with concerns of Placement.
Re-Set for Near Field Listening to ensure that you can evaluate the sound accurately.
Varying the distance between the two mains should help discern a difference.
No personal experience with KEF though all input indicates a refined
highly regarded Loudspeaker.
I think that an Amplifier will make the difference.

I have considered an amp change but really like the size, price, and convenience of the NuPrime. Any suggestions on that subject?

How do you have your speakers positioned? 

You say you're not getting enough bass and you're using speakers that go down to 47hz 6db down. Personally I agree on the subject of metal dome tweeters give me a soft dome any day. But what you need are either bigger speakers with better bass response or a subwoofer.

My room is the same size and have been into this since 2003. I agree maybe a larger 8" two way might be better in regards to frequency response. High efficiency with a tube amp would be even better. I've owned alot of more expensive speakers, but right now I'm having great results the Paradigm Atom v3 (bought used locally for $20.00). These are paired with a $100.00 appj 2013 tube amp and Sangean hdt20. This is the best "complete" system I've had in this small room. I think the source and tubes made the most difference, but the small speakers open up at low volumes and are forgiving in the right way.