No such thing...really. A few companies will cheat their stats or maybe go as far as not to give you any bass.
Set you goal at 85dB, and you will have much better choices and better sounding speakers.
Otherwise you should look at active monitors.
Small is inverse to efficient in speaker design.
Omega makes some fairly high efficiency small speakers (genuine 8 ohm load, efficiency in the lower to mid 90's) using modified Fostex and other fullrange drivers. You trade off bass extension and high power handling capacity.
D. Edwards is right - the inevitable tradeoff of efficiency vs bandwidth means that a small speaker can't give you deep bass and high efficiency at the same time. Whether the Omegas can give you "enough" bass depends on what you're looking for and how much boundary reinforcement the speaker gets. Of course bring a subwoofer into the picture and lack of bass is no longer an issue.
Reimer Speaker Systems, Reference 3A & previously mentioned Omega Speaker Systems are some companies manufactuering fairly efficient monitors that you might look into. Merlin might be another. But D_edwards & Audiokinesis are right, there are only a few unless you are willing to go DIY route & that only adds a few more.
IMO, crossover slope is the major limiting factor. Tubes do not like third & fourth order xovers & second order xovers should be limited to larger push-pull amps rather than flea watt SET amps.
You might want to check out some of the older Paradigm models (3se in particular). I think they were fairly efficient (90-91db), but they were medium-sized monitors w/8" drivers.
Check out the deCapos. Not quite as sensitive as you specify, but the easiest load any amp will ever see (single element x-over). Your max SPLs may be limited, but virtually any amp will drive these speakers to moderate volume levels.
Reference 3A Dulcet
Reference 3A deCapo
These speakers give you tweeters and therefore full harmonic representation of what's going on. Omegas are interesting but aren't wideband enough, IMO, to qualify as monitors. These three speakers will also give you decent bass.
Another borderline efficiency contender that is excellent, is the Silverline SR17-II. The Zu uses a full-range driver from 50Hz - 12kHz, with the tweeter rolled in on a high-pass filter above 12kHz. The Ref 3As have a similar topology, but the tweeter rolls in much lower, in midrange territory like most 2-ways.
The Reference 3A MM DeCapos are 92 db eff, have excellent sounding bass down into the low 40hz region, and have a very simple crossover that makes them easy to drive.
I've used mine with amps as low as about 1.5 w/ch and they were plenty loud in my smallish room. With 5-8 watts they play louder than I can tolerate.
The Reference 3A is a fine speaker and is very popular. Buy them used, try them and re-sell them for no loss if you don't love them.
BTW, mine are the non "I" version.
Thanks for all the info so far. I was hoping to find something to use with one of Don Garber's SET amps. 9.5 watts was the particular one i was interested in. i'm looking for nice, lush vocals. Base isn't too important. My listening room is very small and bookshelf size speakers are really the only option...although they won't be in a bookshelf.
I thought a thread like this could be helpful in the future for people looking for similar match ups.
Zu Tones will be easily driven by the Garber SET, and the midrange is undisturbed by split duties between drivers. They are 95/96 db/w/m in real circumstances but play a little more dynamically than that.
Ref 3a dC or Dulcet should be fine too. I once used 8/8w SET with Dilverlines and SPL was fine in a small room.
Here is a link to Welborne Labs, they list some speakers. No affiliation.http://www.welbornelabs.com/recomendspeaks.htm
Mattybumpkin ; that is a very informative tutoral at the end of the page of your link ! Thanks .
If you can find a pair, the Soliloquy SM 2A3's (probably about $600-$800 with their dedicated stands for the preferred pre 2000 versions) mate extremely well with low powered SET's. They are two way 11 ohm with simple single component crossovers designed by Dennis Had of Cary Audio fame specifically for SET amps(in fact they come with the caveat not to use amps greater than 25 watts). I think they are greater than 90 db. I owned a pair and drove them extremely well with a 1.5 watt per side Korneff 45 amp. If I remember right, they were the hit of the '99 CES show in Chicago (see the reviews)
Reubent above is correct on the 3A DeCapos. I am using Reference 3A DeCapo I speakers (92db) on Skylan 24 inch stands each filled with 50# of sand driven by 9 WPC 300B amps. These speakers will play much louder than I ever listen to music; my room is 14 X 18. These speakers dont need a lot of power to sound good and have a very natural presentation. The bass is very good however I am using a REL sub for a more fuller sound only because I had the sub on hand, but its totally optional. Let me know if you need more information.
Coincident Triumph! either 92 or 94db?