I don't think it's a matter of what's the most efficient speaker (actually, "sensitive" speaker is the correct term); rather it's how sensitive a speaker your amp needs. I'm not familiar with your model of Carey, so I can't say.
The most sensitive speaker of which I am aware are the Avant-Garde Acoustics Trio, which the manufacturer rates at, I think, 107 dB at 1 metre with 2.73 volts input at 8 ohms. However, these retail for well above $20K if I'm not mistaken.
Klipschorns, which are still made, are around 103 dB. Fabulous dynamics, but I find them rather coloured, though people report that they respond well to extra damping and other mods.
My personal opinion is that, overall, taking into consideration performance and price, the best value in a highly sensitive speaker would be the System 15 DMT II from the Tannoy Profesisonal line. They're 98 dB and run very well with a quality 5-10 watt per channel amplifier. At the same time, they have extremely high power handling capacity. Tannoy North America uses Manley Labs 100 watt tube monoblocs to demo them, but you can also use big (200-500 wpc) solid state amps, which is what recording and mixing studios usually do. They also biampable, which is probably how to wring out the last bit of performance from them. The DMT 15s are not sold in high end audio shops---you have to get them at pro audio shops or, on-line, from places like Digibid, though they do occasionally appear on eBay. . .
New they're around US$4K; used around half that.
Another very good approach is the Tannoy 3836 driver, quite a few pairs of which have been available on eBay recently, selling for $450-600, but usually you have to build your own crossovers from the supplied schematics as well as cabinets. But for a modest investment you can put together an exceptional loudspeaker that again, will work well with either low-powered SET amps or monster, high-current transistor amps.
Other options worth considering are the classic Altec 604E, 604-8G and 604-8K coaxials, all around 98-99 dB.
As another poster mentioned, Coincident Technologies offers a number of designs in the 92-97 dB range which have the advantage of much higher WAF than Tannoys and Altecs which do best in large (6-10 cu. ft.) cabinets. They also offer relatively high impedance (14 ohms I think), which is good for tube amplification. These are also very fine speakers, retail ranges from $5K to $8K.
Also in the vintage area, JBL L-300s are 95-96 dBs and are still highly regarded by many.
Finally, if you have deep pockets and plenty of space in your living room, the fully horn-loaded, refrigerator-sized (520 litres), Tannoy Westminsters and Westminster Royals are also 99 dB and perhaps the most emotionallly overwhelming and sonically realistic stereo speakers I've ever heard. List is around $30K with a one-year wait, though occasionally you see used pairs for around $10K.
There are many other options, including the various models offered, at $6-12K, by Classic Audio Reproductions, using JBL and TAD drivers, but I hope this gives you some sense of the wide range of choices.
I myself have long used various models of Tannoys, but that's just me. Others will sing the praises of other makes and models I've mentioned, and numerous others as well.
Happy listening! Regards, Dr_joe
