Single driver speakers. Are they worth considering ?

Mark Audio makes some nice full range speakers.  The large metal drivers can suffer from cone brealup, but the paper ones not so much.  The Alpair 7 metal-cone driver in a "Pensil" MLTL enclosure can be quite musically satisfying.  Don't expect room-filling loud listening levels.  But in nearfield applications, say within 6 feet or so of the speakers with the speakers about 5 feet apart works very well.  Sharp dynamics, great image and soundstage, [obviously] great timing and coherence.  

So, yes.  Full range single drive systems can be quite good.  

There appears to be confusion between full range single driver speakers versus Coaxial / dual-concentric driver speakers.

Tannoy is dual concentric and Kef employs a coincident driver. Both are examples of Coaxial speakers and both (examples given above) employ crossovers.

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.