The Klipschorn is the most efficient one I know of.

In 1960, when I was in high school, we set up a Klipschorn in a building for a Friday night dance.

The place was about 100 feet by 40 feeet, and the ceiling was 30 feet high. There were hundreds of people.

With a homemade 40-watt amplifier, we actually played so loud, and had so much bass, that people were complaining and made us turn it down.

Some of the other Klipsch Cassic designs, such as the Cornwall, are also very sensitive.