Recommendations for best most efficient speakers

Must agree with the misnomer reference. Tube gear when designed correctly carry a very nice punch and can drive well even inefficient loads. Why do you think they build 200 wpc monoblocks? Please get this out of your head.

I took my VTL Tiny Triodes (25 wpc triode) to a shop and drove the crap out of a pair of Martin Logans. The bottom end held together, midrange was what was expected and sweet highs.

Find a decent tube integrated, haul it around and hook it up to speakers in shops. You will not be disappointed.

Pantale,
You might consider a pair of Proacs. I am using Proac 2.5 with 30 watt monos (tubes). Very good and affordable.

If you can go up a few notches, consider Audiokinesis (wish I could afford these), Devore.

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.