While not quite "high" efficiency, I like the Audio Note line for their sound quality, adaptability to a wide assortment of rooms, and wide range of price options. They can be made to sound pretty good in almost any system, but will still reward use in ultra high end systems. While they can play quite loud (for my taste), they are not the best choice for those who like to play at punishingly high volume levels. Audio Note is for those who like a warm, rich sound.
For some seeking a brighter sound and harder edged dynamics, the Classic Audio field coil speakers are worth hearing. But, they are much bigger in size than the Audio Note speakers.
At a recent audio show, I heard a $14,000 Charney system using a single Voxativ driver per speaker (back-loaded horn arrangement) that sounded very good and came in a reasonably compact and nice looking cabinet. The sound was very even (not peaky like most single driver speakers),surprisingly full in the midbass range and reasonably full in the lower bass range. The only thing I would want for more is extension on top.
For lower priced high efficiency speakers, I thought the $3,000 Tekton system I heard at a show was quite good. A Rethm speaker in that price range was also surprisingly good (single driver covering most of the frequency range with a built in powered subwoofer).
The very best high efficiency system I've heard was a custom-built system using field coil drivers from G.I.P. Laboratories. But, the drivers alone have a price approaching six figures.