Single driver speakers. Are they worth considering ?

yes
wideband drivers are great but
subs are needed
a highpass to protect them as well
and many like to add a supertweeter for even dispersion

so its a 3 way, with xo points around 60hz and 10khz. quite ideal actually. probably the best mid ive ever heard came from 1 way.
Ive heard extensively the WE 755a: best mids ive ever heard, spooky good

ps: if you want "fullrange" wideband= high end Headphones

I used 15 Ohm Lowther DX2 in home-made Fidelio boxes for almost a decade, finally just wanted a change.
Still, I am keeping them, as they do some things right; there is even some [illusion of] bass (with a SET amp).
However, they only sound right with that 300B SET amp, anything solid state was poor match (including Pass Aleph 4).

I have not read it in this thread but the biggest advantage of a single driver is linear phase.  It can make what response you do have sound very good and be error free (if the driver is good).  Manger has explored this design idea extensively.  

I am the US office for the Auratone Supercube 5c, probably the most successful single driver speaker in the US, still in production, used in pro for studios.  Entire records (such as Thriller) were built on them, Tom Elmhirst who has built a lot of big records uses them with his ATC 50s..  Very helpful speakers as they avoid most errors.

Dispersion narrows as frequency increases and there is no getting around that physics issue.   Attempts to extend one end of the spectrum seem to reduce the other end.  You can slide the window of reproduction up and down but not wide enough with current materials and designs.  

Concentric drivers are nice like the Tannoy or Genelec, in that they have very little physical offset between LF and HF drivers.  So they image well compared to typical 2 ways.  This CoAx idea has been around since the 20's, I have a Stromberg Carlson 1929 radio with a hand made coaxial driver on a transmission line (the Acoustical Labyrinth).  Its an impressive solution.  

Brad

@tkukielk the speakers look wonderful!