Recommendations for best most efficient speakers

I have used Cabasse speakers (Caravelle, Farella, M2) with tube amps with excellent results. All are around 93.5 dB 1w/1M and work well with the SE and PP tube amps I own. Cabasse speakers are hard to find used. The most I paid was $850 for the used Farella's in perfect condition. Caravelle's were designed specifically for tube amps and are an easy load to the amps. How much power are you planning to use? How big is your room? How much do you want to spend on the pair of speakers? There are a number of choices so if you include those details, perhaps some of us can help you with additional suggestions.

Brad

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.