Jim,
I commissioned the original FTA-2000's and so have listened to them for 2 years now. I'm a speaker guy and decided that the lack of 100 dB+ spls, 20 - 30 Hz response, and dynamics of higher efficiency speakers was worth the coherence and active amp connection that a true full range single driver speaker could provide. I've also listened to transmission line designs for decades and love the full, musical bass they offer.
I love these speakers. I use them in a dedicated listening room that is 8 ft x 13 ft x 21 ft in a nearfield (68 inch equalateral triangular) setup. Like other single driver designs they seem to be at their best with small ensemble pieces (typical jazz, piano, chamber music). The FTA-2000's are tone masters, yet very detailed. And of course they image well. For my 40+ year old ears the beaming is only noticable when outside the sweet spot and then is a gentle roll off. (I can still hear 16,000 Hz.) Personally I listen for the emotive heart of the music, not the analytical details, so these speakers serve me very well.
As GHM points out, these are not your run of the mill single driver design. Most sound thin and lack the bottom octave of music (30 - 60 Hz). With single driver speakers the driver has no place to hide, so get the best you can. The Fostex F200A with it's big alnico magnet is really the key here. The value Brines provides is very high compared to how much others charge (usually about 10 times the price of the drivers, versus 2 times in the case of the FTA-2000). Brines can tweak the speaker for you too (finish, the shape of the black plate behind the driver, maybe even the cabinet shape/driver height). Note that there are only a handful of these speakers in existence.
I also use a RWA modded SB3. And I owned a RWA stock/modded Clari-T for a year with the FTA-2000. The Clari-T did a remarkable job and blew away small tube amps. Yet it could not provide the commanding grip on the speakers that I believe to be optimal. Bigger push-pull amps sounded undamped as you could predict. Brines recommends at least 30 wpc of solid state, but is enough of a "speaker guy" to not concern himself beyond that. I agree with his assessment. GHM, I, and others are using 40 - 100 wpc of chip/digital amplification with great satisfaction. Moving from the Clari-T to Channel Island VMB-1 monoblocks transformed the FTA-2000's from polite diner guests into NFL linebackers wearing tuxedos. In my situations (a 50 year old house in town with original wiring and an electrically ideal new house in the country) I never noticed an advantage with battery power (my SB3 can switch between battery and RWA improved A/C). BTW with the VMB-1's detail and imaging have improved with "small" sounds seeming smaller but better defined (connecting increased detail with imaging improvements). I'd tried a total of 10 amps (including SET, push-pull, old solid state, digital receiver) and the VMB-1 and Clari-T were the best.