Recommendations for best most efficient speakers

A pantale, note that the sensitivity rating alone doesn't tell the whole story, and can even lead you in the wrong direction. A 4 ohm speaker will have a higher 2.83 volt sensitivity rating because 2.83 volts into 4 ohms is 2 watts, but may well not play as loud or sound as good on a given specialty tube amp as an equivalent 8 ohm speaker with 3 dB lower voltage sensitivity. Also, if the amplifier in question has a very low damping factor, the smoothness of the impedance curve will be a factor in the net tonal balance.

Duke
dealer/manufacturer

Unless you define what "cost prohibitive" means to you it is unlikely you will get a good answer. I was going to recommend Avantgarde Trios with multiple Basshorns but they are prohibitive at over $100,000 so I recommend the much more reasonbale Duo Omegas which are closer to $25,000.

Zhorn, Omega, Fritz, kcsloudspeakers, a small sample of loudspeaker manufacters offering sub $1000 med eff [arround 94db 1 watt 8ohms]loudspeaker systems.

There is a nice write up over at goodsound.com on the main page.

tekton, hornshop got to be more, anyone else care to add to a under $1000 pair list?