Ring radiator tweeters - the future?

Gmood1...The frequency range chart that you cite is for the fundamental tones. Harmonics do go to 20KHz and even higher. But I do agree with you that the low and mid range is most important, and a bad tweeter is worse than none at all.

Gmood1...Why do tweeters exist? Because drivers with large cones, necessary for LF reproduction, are lousy for HF. "Lousy" means irregular frequency response within the pass band (peaks and dips) and premature roll off, sometimes covered up by a kluge called a whizzer cone, which generates lots of HF sound not necessarily related to the music. (But the ear is fooled). This can be worse than any effect of a crossover. A tweeter, having no requirement for LF response can quite easily be optimized for HF. With proper crossover design, and a tweek or two, use of a tweeter is the best solution for full range sound.

If you are happy with response to 10 KHz or so, then you don't need a tweeter.

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Gmood1...I assume the the Fostex F200 is what you use. Good choice. No whizzer.

But look at the almost 10dB hump between 4 and 5 KHz. And how about off axis? And you can't really tell about the HF (15 to 20KHz) from a SPL measurement. Sure it puts out loud noises at those frequencies but is it part of the music, or just a rattle? The ear (at least when you are young) senses that there is HF sound present, and "hears" what harmonics ought to be there. The ear is fooled in a way similar to the eyes that sometimes see what you expect (a dog perhaps) instead of what is really there (a tree stump).

Of course if it sounds good to you it is good. Maybe your ears have a notch at 5000 Hz.

The FR driver in a folded horn enclosure was commonplace when I began this hobby in the mid fiftys. I had a really nice wharfedale 8" driver in one at the time. But I never liked horn resonance as a way to boost weak bass. Or vented boxes either. Or boxes of any kind.

So after telling you all the things I find wrong with FR drivers it may surprise you that I am seriously considering some experimental work with one. My idea is to exploit the extended range capability by using a FR to cover the range from 100 Hz to 8KHz or so, with a SW below and a tweeter above. This keeps the crossovers out of the critical range. Of course it will be open baffle, and I may employ multiple drivers in a line array. Most normal woofers start to deteriorate around 2KHz, with a crossover required around 2500Hz.
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The so called "tweeter" of the Magneplanar QR series, like my MG1.6QR, is not a physically separate driver, like the ribbon used in the MG3.6. It is just a section of the diaphram that has lighter conductors and can vibrate at higher frequency. In some ways it is like a full range cone driver, where breakup of the cone allows the area near the voice coil to vibrate at higher frequencies than the outer cone area. I actually prefer the Maggie QR over the ribbon, and the money left in my pocket is nice too.

Gmood1...The Fostex FF225 appears to be much flatter out to about 12KHz, where it drops off completely. At the low end there is a rolloff starting around 200 Hz, but it is smooth and gradual and easily handled by equalization down to where a SW would take over. This driver, which costs about 1/4 of the F200A, is a better fit to the requirements of my project.