Speaker db rating?

Don't forget that the crossover used can affect how much power you will need, a complex crossover with lots of parts will need more power to sound dynamic than a simple or no crossover will. Some speakers have only a cap on the tweeters while others use no crossover(single driver designs) Tell us which speakers you are considering or what is your price range and I'm sure you will get lots of suggestions. For low cost try Hornshop Horns, Reference3 A Mm De Capo's will work with 8 watts also (92db@1watt1meter and only a cap on the tweets. I use Sonist Concerto2 (95db rated with good results paired with 5 watt amp.

Groundhog- Read this before you get too confused between amplifier output power, SPL and perceived loudness levels: (http://www.audioholics.com/education/frequently-asked-questions/relationship-between-watts-and-dbs) You'll notice that it actually takes 10 times the amplifier power to produce an SPL that SOUNDS twice as loud to our ears. That is an important factor to consider with your goals.

This site is a bit more in depth in it's presentation of the facts: (http://www.analogrules.com/dbwatts.html) The tool that Hemihorn posted takes these factors into consideration.

For simplicity:
Assuming 92db speakers
1 watt = 92db
2 watts = 95db
4 watts = 98db and so on...

Takes double the wattage to increase 3db
Takes 10 db to have perceived doubling of volume

Please forgive me for getting a bit off topic, but, with all due respect to JohnK; it doesn't make the case for hi-eff horns for me.