Single driver speakers. Are they worth considering ?

It’s all about where you’re willing to compromise. I have a diy 6.5-inch ported bookshelf speaker that I think sounds great — for what it is. The mid-range is just very sweet and the highs are good. But I added a subwoofer soon after. Looking at my speaker measurement w/o the sub, I’ve got a nice 100Hz to about 8 kHz spectrum. That used drivers costing $100 each, so not cheap-o but not fancy either, with neodymium magnet and a cast basket, so good construction. And I tuned them pretty carefully with acoustic stuffing and port size. Put on girl-with-a-guitar music, or “unplugged” Clapton or Nils Lofgren, or moderate-level jazz or combo, or chamber music or piano solo, and they do great, a real pure sound. And think about it — that’s a lot of great music! It’s probably about the time domain and point source and other stuff I’m not competent to address. Ask Atmasphere. And they were 95 dB sensitivity for the 15-watt class A amps, so a good team. But not as a do-it-all speaker, I’d say.

Feel the most famous single driver speakers would be the Bose 901. Yes they had multiple drivers but all to form a single driver in basic delivery. What was the quote “ no highs no lows must be Bose”. I did and still have a pair. Need an amp that had a power supply that could double as an arc welder to drive them properly. Sansui 9090 Them run through a eq to shape the sound by bending the wave form to an extreme level. Yes set up right and playing loud to an over served crowd they sounded good. Set up with a quality system in a listing room you keep struggling to try and hear what is in the music you know is there but you can’t hear it. I have never heard ZU speakers but they do have a good reputation and following——

I have an 8 watt 300B system (Wavelength Duetto), and single driver Stein Music SP 1.1 speakers.  I have very much enjoyed the Steins for going on five years (a record for me).  I cycled through many traditional speakers prior to them, and even during this period have scratched the itch to buy and try Devore 3XL, 0/93, Daedalus, and Audio Note AN/J. 

I do of course notice less bass response as well as a loss of some treble.  That is why I thought I might be able to improve upon them.  And for a week or so every one of those trials convinced me that I made a good choice.  Then, I put the Steins back into the system and very quickly decided that the Stein's presentation is more organic, realistic etc.  The immediacy of single driver speakers is very addicting.  For me they give me a "band is in the room" vibe better than the other designs I've tried.  Holger Stein is known for effective, natural and physics-based tweaks and his SP 1.1 design considerably tames any downsides to the design. Your mileage and ears may vary.

I just recently found a set of Musical Affairs speakers with natural wood cabinets and PHY Sag drivers, and am looking forward to my 2nd pair of single drivers!

I have a friend here in Vegas with the AER BD5. It is an insanely expensive driver ($45K/pr) but the sound is incredible. I need to add that is is augmented in the bass as it is on a 200hz LeCleach horn but absolutely stunning regardless. I have never heard a single FR driver sound that good. Everything else has some issue somewhere. If you do not have the budget or equipment then you have to accept some compromise somewhere.

I once owned a pair of Rethm speakers with Lowther drivers. The problem was a thin sound with no bass. Adding a Rel subwoofer didn't help much. There was always a dip between where the driver left off and where the subwoofer picked up. 
Check out Bache Audio. I have the Tribeca speakers.  These are based on a modified wide band driver that covers the range from 700-10K, augmented by a super tweeter and 2 woofers. The wide band driver is directly connected to the amp (no crossover). It's like a single driver but with better top and bottom end.