Don,
I've heard many single driver speakers, in various cabinets. In nearly all cases I prefer to call them extended range, not full range drivers, to avoid quibbling and to accurately reflect their performance. Another qualifier, I don't consider coaxial designs (tweeter at the center of the woofer) to be a single driver. Regardless of what you call them, they all have the following advantages:
1. Time and phase coherency - no crossover, no physical offset of multiple drivers, no differing sounds from various (types) of drivers. Ideal point source for imaging/soundstaging, especially under nearfield conditions.
2. Direct amp to driver connection - amazingly more/deeper bass and more dynamic with the amp being able to "see" the load for clearly.
3. Mid-range emphasis - the middle 6 octaves (80 - 5,000 Hz) is the essence of music and at the heart of what these drivers do best.
Some extended range drivers, like Lowthers, are expensive. Most are highly efficient (size being a factor). Many extended range drivers, like Lowthers (which have been around for 70 years), use whizzer cones (smaller secondary cones) to extend treble response/dispersion.
In order to extend bass response (the primary shortcoming of extended range drivers) various, fairly exotic, cabinet designs have been developed such as front loaded or rear loaded horn designs, tuned quarter wave pipes, and mass loaded transmission lines. Conversely the "simplier the better" camp has applied them to open baffles (just the front mounting board in several variations). The obvious challenge here is lack of bass response.
For a smaller system the 4.5 inch Fostex FE127E is a solid performer. Fostex offers several extended range drivers and so is probably the most popular source for these types of drivers. You can buy a pair for $75 from Madisound and try them in simple open baffles and if you like them, send them to Louis at Omega to be installed in one of his very fine cabinets. BTW Fostex also publishes deceptively simple but highly tweaked cabinet designs for each of these drivers if you'd like to DIY.
TBI does have a good reputation for their very unique subs. The Diamonds seem to use similar technology but aren't well known. Most direct purchase vendors offer a trial period.
Before you jump at a Sonic Impact amp, I'd check out Partsexpress and look into the Sonic Impact Super T amp that for $100 more provides lots of tweaks:
http://www.partsexpress.com/pe/pshowdetl.cfm?Partnumber=300-956&raid=27&rak=300-956Here's a review of it:
http://6moons.com/audioreviews/sonicimpact2/t2.htmlAnd to complete the little system, I'd recommend a $300 wireless Squeeze Box. It allows to you rip all your music to your PC hard drive and playback very conveniently anywhere in the house. It includes volume control, software, and internet radio. The Squeeze Box is 8 inches wide, 4 inches tall, and 3 inches deep. And it can be modded later on for extremely good sound.