Single driver speakers are not without flaws, but offer greater coherency, immediacy and purity than multi- driver speakers. Probably due to the elimination of the crossover. Generally speaking, the smaller the driver the better the highs and the larger it is the better the lows. Of course there are ways around this like using a sub or supertweeter. Or both.
Solid state will work and are actually preferred by many single driver speaker owners, especially in Japan. Those using SET amps need efficiency (Lowthers in rear loaded horn enclosures).
Caveat: Once you get used to the full range driver sound, you will begin to hear the crossover in multi- driver systems.
Check out: fullrangdriver.com
"Single driver" speakers are rarely full-range: it's (as yet) impossible for a single transducer to reproduce 20-20kHz. HOWEVER, the reference to "single" driver is in the use of one drive unit reproducing all or most of the ear-critical region, i.e. ~100 - ~10kHz (or 300-6kHz). The sound benefits greatly, as Ultrakaz notes above: you only have one source for the bulk of music's frequency content. And, therefore, you do not need a crossover in that area, AND the result is very versatile as, when you DO use a-over it can be behind the amp (line-level) - OR at frequencies where our ear is less sensitive to distortion effects (150Hz, etc).
On the down side, there are small peaks in the amplitude response as you go up in frequency (that can be addressed with appropriate filters, if need be), good wide-range drivers are expensive, accompanying super-tweets and woofs/ subwoofs MUST be commensurate (i.e. very good quality).
Good "single driver" designs are very high-end, both regarding reproduction & price.
Using ss is not a problem IMO -- I'm happy with it (I also use SET).
The ones I heard briefly at shows sounded bad to my ears. If the composer called for thunder, by jove the speakers should put out thunder. They all sounded to me like listening to a table radio, a good table radio mind you, but a table radio nonethelss. Unfair to judge anything on a short listen in poor conditions such as shows, but nothing twigged me into wanting to hear more. A cult item if ever there was one.
I tried the single driver route. My experience was that I heard some of the benefits and I also heard the problems. They can sound very clear and coherent and,most importantly, involving. I couldn't, however, get over the obvious reduction in bass output and some of the unusual tonal balance problems. Maybe with much more expensive drivers, like Gregm suggests, the strengths would be greater and the problems less obvious. I used $280 Fostex drivers. If you were to buy a subwoofer,add a supertweeter and use filters for the frequency response irregularities, it could be made to work. It's a mixed bag of strengths and problems. Can be made to sound glorious,see the TNT article on the use of Lowther drivers with success.
Now, here's a twist on single driver speakers. My Acoustat 2+2s are a full range electrostatic going down to 28 Hz. All panels are full range, so no crossover distortion. They are very fast speakers and at almost eight feet tall they have a terrific stage. I'm driving mine with a Bryston 4BST.
"Full Range" cone drivers are not really "single" drivers. "Cone breakup" occurs, so that different parts of the cone are radiating different frequencies. Design of such a driver is more of an art than a science. The breakup characteristics of the cone constitute a crossover network...a mechanical one rather than an electrical one, and much harder to design.
The only real advantage (IMHO) of full range drivers is that the high and low frequency transducers are colocated. However, this is also true of a coaxial speaker, where the tweeter is mounted where the cone dust cap normally is located.
There are some applications, eg: speech, where high and low frequency response is unnecessary, and, for these, a good full range speaker is ideal.
I have become a big fan of single driver speakers. The Lowther and Fostex drivers sound especially good to me. There is something very, very right about the sound. What I like the most is how voices and instruments jump out at you, just like in live music. The speed and clarity of these drivers is also breathtaking
Currently, I have two pairs of Lowthers, and have cut up the lumber to build a large cabinet. I was wondering if I built the cabinet too large, but hearing the speaker that was paired with Audio Note Japan in NYC made me very happy. I will try both Lowthers, ceramic and alnico magnet, and probably keep the pair that sound better in my room.
I never thought these drivers were supposed to mate with solid state amps, but the Red Planet and Rethm combination in NYC had very good sound. Despite the comments that the Rethms have no bass, I found the sound very rich and full sounding.
Eldartford makes a good point regarding cone breakup but, if properly managed, breakup also increases dispersion, resulting in much less midrange beaming. Most two ways also operate the low frequency drives in breakup mode in the mid and upper mid frequencies. Sometimes it is beneficial to trade phase response for improvements in other parameters, but it does mean that most single driver moving coil systems are not phase coherent. At least not if you define it by meaning that the driver can pass a clean square wave.
Nothing's perfect. I like my single-driver setup just fine. I've had alot of the "more normal" stuff, and the single-drivers sound better to me, especially when using a low power SET amp.
Martin Logan CLS: Probably one of the best single driver designs ever made. A total steal too, on the used market.
Owned Cls2z many other logans much perfer fullrange drivers systems usually with a super tweeter.So they are not truley fullrange! also most [fullranges] need help with bass so you end up with a 3 driver system !only Fullrange I have used or heard with a fullrange sound is fostex f200a it has all the bass 1 would need I get a usable 30hz-21khz .Owned many other FR drivers.All need help in 1 way or another except f200a..Just my 2 cents worth