Very small room systems and synergy

I have played with most other aspects of the system.  I've moved speakers into every possible position including moving furnishings, I have acoustical treatments that I've played around with for years and I think that is good now.  What I find is a lack of overall frequency response.  I'm 63 years old so my hearing isn't what it was but all my systems have sounded screechy and lacking in mid bass to bass.  I've even had subwoofers in the past I used with Maggies.  Maybe I expect to much?

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.