You don't offer a price range, so I'll assume that you have a decent budget. Generally speaking, the most efficient speakers available (in the 96db+ range) are horn speakers. The Avantgarde Uno and Duo have gotten very good critical comments, and they will play VERY loud with low amplification. Of course, there is always the Klipschorn. While not considered an audiophile speaker, it provides pretty decent sound and it will provide prodigous decibel levels with little wattage. The Coincident Speaker Technology line of speakers is also being heralded as a great match with low watt tube amps. While they are rated in the 92 db efficiency range, the tubaholics seem to like them a lot.
High Efficiency Speakers
I'm building an affordable all-tube system, and I need to start with very efficient speakers so that I don't have to spend $$$ on a SET amp. I'm targeting a 2.5w SET w/ 2a3s tubes from Bottlehead. Consequently, I need 96db+ speakers. I primarily listen to accoustic guitar, classical, and vocals. What speakers would you guys suggest I listen to?