Place "Fostex loudspeakers" in a search engine. Then do the same with "High efficiency loudspeakers".

I am using a Japanese T-Amp (5W @ 8Ohms) to drive my Bose AM5 S2 so I would think they are efficient enough. Build your amp and try them out. Coral Speakers are an alternative to Fostex. A friend of mine has Coral Flat 8 in a horn enclosure which are driven by a 0.75W valve amp to decent listening levels.