Single driver vs traditional 3 way loudspeakers

The best MTMs I've ever heard were ribbon/planar/round

OK now we are talking 
a  ribbon might work, vs a  midtweet. 
I said might..
I'd have to hear a ribbon mid tweet vs the 2 SEAS top  mid-tweets. 
Which are the Cresendo and my Millennium. 
Thing is, tell me ifa  ribbon has that same liability/weakness as a  planar..which is a narrow dispersion,,which i know is the case,,, which is why 1 speaker design using ribbons has 3 in a   straight line row for just that reason, = 3 ribbons = $$$$$ vs Milleniums  $250/Cresendo's $300.
3 good ribbons = 300 each thats $900 and still that narrow sweet spot would just grate my nerves. 
 

 low power SAT  

OP, mentions a SET amp, = I hate SET amplification. 
SEAS does make high eff speakers but ya gotta pay the piper. 
SET amps can not play complex/full range orch music with authority,,,SET amps are for light jazz only

Love this topic!

FR speakers certainly have their limitations. They have a very specific sound signature that may not be to everyones taste. I've built a couple, for myself and friends. My "Pitch" to my friends if they already have something good, but a bit "generic" in their system, is "these will sound very different than what you have, why build another like the one you already have?"

They breathe in my experience of them. There is an openness and airiness that comes through quite beautifully. I find them not fatiguing in any way and can have them on all day.

Running a single 4" driver, I don't find I'm missing much from the manifold horns I built. An 8" pair I built for my friend really kick!!! 105db and no strain whatsoever. Granted, it was harpsichord music, but it sounded effortless. As did the Gregorian Chant we were playing at similar volume. The "Serious Fun" by Lester Brown, which is not an easy recording to reproduce, came across beautifully!!! No strain, nothing out of place, just nice (albeit loud...)

And I did tell my friend, that if we get complaints and the neighbours call the police (we were playing this at 1am in a residential area downtown) I would just leave and let him explain the harpsichord being played so loud.

I'm a fan for sure. Will be transitioning to Coax drivers I suspect, using a similar cabinet design, but utilizing a crossover for the tweeter.

Here is a link to some I've built:

https://www.frugal-horn.com/downloads/SpawnHorn-v2-planset-181117.pdf

Also built these, still to be completed...

https://www.fostexinternational.com/docs/speaker_components/pdf/recom_enclose/108ez_encl.pdf

All this to say, I like low power, single FR speakers...

It’s better with a three way!

I have Maggie’s in two other systems, but I built a single-driver speaker after building the Pass Amp Camp Amp (8 watts class A). I was so pleased with the amp’s pure sound that I built another, and then a Pass diy Balanced Zen preamp. Each of these use minimal components and simple circuits with the goal of leaving the signal as unmolested as possible. So the next step in the simplicity chain was the speaker, diy of course. The result, after experimenting with sealed vs. ported and various degrees of acoustic stuffing, was a 6.5-inch “full-range” driver in a smallish bookshelf, ported enclosure.

I went with a 95-dB Dayton Audio driver from Parts Express. It had a neodymium magnet in a cast frame and other recommended attributes, and then a Kevlar/paper cone with a whizzer and phase plug. It’s pretty good sound, similar to the Boston Acoustics HD5s they replaced but much more efficient. I later added a Martin Logan 400 sub, since my Dayton mic and sound software confirmed a pretty steep roll off below about 100 Hz. 
It’s all a trade-off. No crossover, but not “full-range“ either. Small enclosure, but I didn’t have room for larger. Still, in the all-important midrange, it’s very good. And that’s my diy “simplicity” setup.