Wide dispersion + high efficiency + detail + coherence in a speaker?

You should should check out Living Voice OBX RW speakers. They are effecient/sensitive with a benign impedience curve. 94db and the impedience does not drop below 6 ohms at any point. In a small room, 12x14 or so,  8 watts is fine. In a larger room 20 - 30 watts will do wonderfully.

They are the most coherent speaker I have owned and deliver uncanny musicality. Wonderful wide sweet spot and soundstage to boot. The midrange is very open and lively with wonderful tone.

The bass while not 20 hertz deep and thunderous is full bodied and present down to about 30 htz. They are rather small and easy to set up in most any space.

Depending on your budget and on the physical size of a speaker that would be suitable for your room you may want to consider the Daedalus Argos. Click on the "specifications" box on the left side of the page for detailed specs.

As an owner of their Ulysses model, which differs from the Argos mainly in that the Argos is designed to provide a much wider sweet spot, I suspect it could very well meet your requirements. I don't know what current pricing is, but as a very rough guess I wouldn't be surprised if it is in the vicinity of $20K.

Good luck. Regards,
-- Al  

Charney Audio horns. I have the Maestro with Omega RS7 drivers. All of his horns are high eff and produce clean articulate coherent bass. Charney utilizes the Tractrix theory in his designs. The horns are rear loaded and couple with the room to produce coherent, 3d,  and engaging music. No sub needed for any of his designs. I have had a number of people in my room (11x15x8) and usually the first response is "How did he (Charney) get such great bass out of a 6.5" driver?".
Zuio - All of your speaker requirements are delivered with Charney Audio Horns.
http://charneyaudio.com/  

The Zu line of loudspeakers have nearly all of what you are looking for.  I don't believe they have the wide dispersion.

@zuio --

Perhaps some older Tannoy Gold 15" dual concentric drivers would do the trick. Simon Mears Audio makes Tannoy Autograph replications with these drive units, refurbished and all, and they provide you a uniform dispersion pattern (much more important than "wide" dispersion, I’d say), true high efficiency (~99dB), detail/insight, coherency in spades, and great dynamics and bass to boot. They’re big and heavy, and will cost you at least or about $20,000/pair, but you asked for it without specifying a retail limit, and I’m giving you an example of speakers that may fit the bill according to your needs. Not least, they’re beautifully made from ground up to original specs.