+1I've never found a full-range driver to be as good as multiways.
Firstly, no ''full-range' driver is ever full range. If they are big enough for bass, they won't do treble, both from the sheer mass of cone and beaming of treble like a flashlight.
Then there's cone breakup. To have any chance of doing treble, the cone must be light, or have a HUGE magnet system, and light cones tend not to be so stiff, and large magnets are very expensive.
Even panel loudspeakers like electrostatics, which may the thought of as 'full range' have different sections for different frequency bands, as much to reduce beaming as anything else, but even with the lightweight diaphragms used in electrostatics, mass matters at HF, and output and sensitivity are related so an electrode spacing that's good for bass won't be right for high frequencies.
I suppose it's possible, using exotic magnetic and cone materials, to have a driver that's nearly 'full range' but it'll be fragile and expensive, still won't perform as well as a two or three way, and so will be a poor engineering solution.
S
The best compromise between two speakers designs I think:
* A small coaxial like KEF 5.25" (KEF 6.5" sounds like traditional 2-way, without the "magic").
* Woofer up to 6.5" and... a very good waveguide in the tweeter. Bigger room, you can try with bigger woofer.
Always two-ways, not three ways, to listen to very good recordings.
But in earphones and headphones, a single unit is better. This is because earphones and headphones operate in a pressure chamber. In pressure chambers, the diaphragm displacement at low frequencies does not have to be as large as the free sound field. Sound pressure is proportional to the displacement.
Back when my wife lived in Japan, and I used to visit often. Akihabara had many small shops that catered to DIY single driver full range hobbyists. They'd always be demo'ing some female vocals/jazz, and at times they really did sound special. They must be pretty efficient too, since they're often powered by low powered tube amps.
I wonder if these things were popular in Japan due to their tiny living quarters. These shops were basically booths barely larger than your average office cubicle. My wife's old apartment was the size of my garage.