Single driver speakers. Are they worth considering ?

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. 

Unless your budget is infinite, there will be compromises.   So the question becomes, what are you willing to give up ?

A single driver can deliver deep bass, lifelike midrange, extended treble, but not at the same time.  It is physically impossible to create a single driver that can deliver 90db of distortion free 30hz bass, AND deliver 90db of non directional, undistorted 15khz treble at the same time.   So what are your preferences ?

The majority of musical content lies in the upper bass to low treble region....about 100hz to about 5khz.    That range should be achievable using a single driver system.  One can add loading to extend bass response another 25-30hz, and an outboard tweeter for anything above 5khz.  This would technically become a 2 way system, but the vast majority of sound, including the human voice and almost the full range of a piano, will come from a single driver.   

The famed Western Electric / Altec 755a excels in this application, and many say that modern drivers still do not come close.    Caveats are the fragility of the driver....15w is too much power.  That said, the driver is very efficient and can deliver room filling sound powered by a SET amp. 

Single driver systems sound different than mulit driver systems.  There is merit in point source reproduction.   (This is one of the reasons why people like small monitors also).   Some have gravitated to coaxial drivers as a way to get past inherent limitations.  Vintage Altec, Jensen, Tannoy drivers  are valuable and held in great esteem because of the clarity of their sound.  But to get the best sound one has to invest in a precise outboard crossover, careful amplifier selection, and construction of a cabinet well matched to the characteristics of the speaker.

Be prepared to experiment, and think seriously about what you want to accomplish.