Why Hardly Any High Efficient Bookshelf Speakers?

Yep there is - their size. Small resonance chamber makes for a tough load in general. This is especially true for "regular" cone type speakers. Whizzer cones and horns do much better.

I owned a pair of Omega TS1s and they were very nice although a little thin sounding in general - not a fault really, just their character. They were neat speakers with infallible build quality. Good luck though - the Omegas were what I ended up with after asking the same questions as you! Arthur

Hi Tb:
Here is a short article.
Basically, if you want small and a little bass too you have to give up SLP/sensitivity.
http://www.trueaudio.com/st_trade.htm

I remain,

Efficiency is determined by the size of the speaker driver and the size and design of the enclosure. True efficiency is a balanced frequency response from at least 50Hz on up. Generally, this means that most 8" drivers will produce 50Hz at approximately 90db at one watt in a bass reflex cabinet. A 10" will do 93db at one watt and a 12" about 96db at one watt. A horn loaded speaker will produce more bass but the cabinet will be quite large.

Therefore, if a bookshelf speaker with a driver less than 8" has a published efficiency rating above 92db, it will most likely have a rising midrange and sound bright and tinny.

Klipsch makes rather efficient bookshelf speakers.

Look at the Galante Audio Rhapsody. It's not "smaller than a breadbox" but it is not as big as a horn system. Bass supposed ly to 50 hz, but I've not heard them; I have the larger version, the Symphony, which is 12 inch (98 db), rather than the Rhapsody's 8 inch (95 db). These do go pretty low (maybe 40) and sound great with a 2A3 amp in a "live" room. Brian Galante himself answers the phone, so service is first rate. Both models are occasionally for sale here on A'gon, which is how I got mine. They have a website: galanteaudio (all one word)