Single driver loudspeakers have their charms and their shortcomings. There is no perfect loudspeaker. If you follow Floyd Toole you'll know why (inevitable in-room bass peaks/dips from using side-by-side bass sources). Coherent presentation is one of their strongest suites, especially if you listen near or mid-field. Being point sources imaging is another strong point. Another advantage of single driver loudspeakers is that by default they are "active" (one channel of amplification per driver) and thus share that advantage.
17 years ago I commissioned FTA-2000 from Bob Brines (single driver floor standers that use a transmission line ala Martin King). Bob has since retired. The FTA-2000 uses 'the mighty' Fostex F200a driver (8 inch diameter, rated 30-20,000 Hz, 8 ohms, 90 dB/w/m, 80 watt peak, very well made, AlNiCo magnet, no stinking whizzer cone so it "beams" above 4,000 Hz, when last available cost $575 each). Not your ordinary thin sounding Fostex driver, this one is rich and full bodied. Some knowledgable folks have called it the best single driver in the world, when given the EnABL treatment which I paid Bud Purvine $600 to perform.
Years later Duke LeJeune, from Audio Kinesis fame, suggested adding "Late Ceiling Splash" tweeters (just $20 1 1/8 inch soft dome with a cap parallel to the loudspeakers placed directly behind the loudspeaker and pointing straight up (which provides enough delay so the ear isn't confused). They added treble response, ambience, and expanded the sound stage. And thanks to Earl Geddes who echoed Toole I've added 3 subwoofers. So what I have is at setup that is wonderful for small ensembles at limited spls, but is a hybrid single driver loudspeaker. Along the way tried JBL 708P ($4000/pair active 2-ways) that all around were wonderful performers but had their own short comings.
But getting back to single driver loudspeakers, most lack bass due to limited X-max (plus many being relatively small), whizzer cones are simply mechanical crossovers, larger drivers "beam", most have limited spl output, and many have poor treble performance. Most are high efficiency, which is another reason why they have limited bass response, in fact many SET fans actually dislike deep bass (an interesting phenomenon). Coaxial drivers are only improved upon versions of 2-way designs, complete with crossover.
