Single driver speakers. Are they worth considering ?

I've never heard a full range single driver speaker, what's a good example of one for the OP to consider? It would need to produce listenable sound from 20hz to 20khz. Reasonable SPL and low distortion. 

Full range driver impossible to design and build ?

I'm interested in Cube Audio's cheating version.  Full range with subs with an 80/100Hz crossover.  I might build a version of it for myself.  You can buy the drivers and put them in a sealed box for under $5500 total cost.  Then I'd use the 2 x 12" cabinets from my Coincident PREs for the lowest registers.  Maybe you lose a bit of the magic of the full range driver by incorporating that crossover, but you also get full extension that way... and no crossover in the most important regions. 

Build them with Hypex plate amps and use DSP crossovers you won't lose any magic just gain a better speaker.

TANNOY dual concentric driver. They pioneered the technology long ago, 1947 to be exact. 
https://www.tannoy.com/innovation/Dual-Concentric-Driver-Technology.html