Single driver speakers. Are they worth considering ?

The Rethm speakers I’m referring to are the current version that does not use the Lowther drivers as mentioned in the post above. I do not know the brand of driver that is being used, but there were none of the issues that jdal mentioned in his post. But the model speakers I’ve heard were the Saadhanaa’s.  Very clean, strong and full bass.  

One particular factor is a very difficult to accept reality. The range of heard frequencies declines with age. Modern noisy life certainly has exacerbated that too. By age 60 everyone has lost some significant degree of high frequencies. If you can connect a computer or other internet enabled device to your system, google for a frequency generator and sweep your system. If you are over 50 you will certainly hear a point in the low teens where sound disappears. 
Once I had done that, despite all my resistance to it, I realized that at least when it came to frequencies above about 12 or 13 kHz, I simply could no longer hear them. That made the idea of a single driver wide band or full range speaker system much more acceptable. One of it's flaws is matched by my own.

One designer builder not yet mentioned but renowned for exceptional full range drivers is Oleg Rullit. His designs deliver superb sound. His 9" and 12" Super Aero field coil drivers when properly mounted in front loaded or open baffle designs really lack nothing with most music. Supremely musical with surprising bass and smooth response right up through my hearing limits. 

I have a set of Markaudio Alpair 12P set in a "pencil style" cabinet with appropriate internal organs and ports of 1" B grade Baltic birch on 3 sides.  The front baffle is 1 that's right 1 solid piece" of 1 !/2" of FAS bookend bird's eye maple.  Near field they are fantastic.  They are not for everyone, more of an extra to your systems.  The speakers are extremely complex, fragile, difficult to make, it's pretty neat to read how they are made.

Absolutely not. Why would you even consider? There’s a really good reason that the large majority of audiphiles buy speakers with more than one driver. It’s not dumb luck or brainwashing. It’s that multidriver speakers sound better. I know there’s a few guys that speak about a single driver that’s “full spectrum” in the hearing wavelengths that make listening to music so enjoyable. Well maybe their ears are satisfied with a single driver, but at almost 70 y/o, a musician, and a man with numerous friends in the music industry, mine are not and neither are theirs.

Personally, I am not willing to compromise. Everything that has been mentioned in this thread that I have heard has been hopelessly colored.
Some people like the sound of these speakers. So I suppose what colored loudspeaker you like is a matter of taste. 
The only type of loudspeaker that can approach full range performance and avoid coloration is an ESL which the OP did not want to hear about.
Sorry. Just the way it is. Dynamic drivers are flawed in so many ways it is remarkable that some systems can sound as good as they do. Full Range? This is an illusion.