Single driver speakers. Are they worth considering ?

I am still running a pair of Cain&Cain Abby's that use a Fostex single driver and a Voigt pipe design. They have kept pace with my other upgrades, and I love them. 

Although their bass is surprisingly good, there's nothing to stop one from adding subwoofers; I also recently added rear-firing horns with exactly the aim that Ralph described in his post: "The solution is [... to] add a rear firing tweeter (and not cross over the main driver) to correct the tonality and help with soundstage palpability" (both products I purchased from Audiokinesis).

These two additions solve many of the issues with extension while preserving the coherence and liveness of single main driver. I am sure many members here have superior set-ups, but OP, you'd be surprised how full and immersive these speakers can sound.

Bache audio and Tri-Art both pursue designs built around a full-range driver supplemented by super tweeters and woofers.

dht4me,

I have not heard the AER BD5 driver, but I would bet that it sounds terrific.  I've heard the BD2 utilized full range (no other drivers) in a couple of systems.  One just had a large horn on the front, and was completely open in the back. This system had a very pure sound, but no deep bass.  The other was a Charney speaker that had substantially more bass because of its use of a backloaded horn on that driver.  The Charney speaker is fantastic.  I've been somewhat interested in the BD5 in a backloaded horn system.  AER sells a very expensive system that utilizes that driver in with a front horn/waveguide and a backloaded horn for bass.  It is an interesting looking system, but, it is quite expensive.

I've also heard, and liked, a full range Feastrix field coil driver.  The system was a bit deficient in bass, but, the driver was in a fairly small Jensen Onken cabinet, so I don't know if bass response was optimized.

@sheldonbsmith, at the risk of appearing petty, though without electrical cross-overs, the Ohm Wash driver does have mechanical cross-overs, unlike the similar German Physiks DDD driver, though the DDD is not full range.

@realworldaudio, with proper application, time aligned drivers coupled with first order cross-overs can resolve the time and phase issues that typically plague other multi-driver configurations, and lend themselves to full range capabilities while doing so.

Horn loaded 7.9" Field Coil Driver.
14.4" wide x 39.4" high x 17.7" deep cabinet with a rectangular front facing port.
8 ohm impedance.
96 dB Sensitivity.
20 Hz to 20 kHz Frequency range.
$20k (in 2018), standard finish and $21k, upgraded finish.

The Shindo Lafite.

I love how easily the Lafites disappear and the wonderfully musical image they produce. They sound good at low volumes and at elevated volumes that easily pegs my iWatch’s SPL meter. I’ve had a pair of Lafites for several years. Most of the time, I drive them with a 10 watt/channel Shindo Cortese SET F2a power amp and Shindo Giscours preamp.  Using analog or digital source material, the Lafites do not go as high nor go as deep as a very nice sounding pair of 25 year old Vandersteen 3A Signatures driven by a VTL ST-150 power amp in triode mode with KT88 output tubes and with an old faithful Conrad Johnson PV-11 preamp.

As mentioned before, it is so easy and cheap to find out, why not try it. Pencil, or Frugalhorn, or Woden - these designs give a pretty flat response from around 40 to 14,000Hz, are really easy to drive, easy to adjust to your room and tastes - and if you don’t like them there is a market for them if your DIY skills are halfway decent. The money you save can be spent on music, or fancy cables, or whatever.