Generally speaking, sensitivity is measured by driving one watt through the speaker and measuring the relative volume from one meter away. The "loudness" is measured with a SPL meter and indicates how sensitive the speaker is. More efficient designs will have 90+ specs and are easy to drive, making pretty well any amplifier suitable. The most efficient speakers are horn speakers, like the Klipsch's. From there, it's all over the map, whether the design is bookshelf, floorstanders, or panels. Bottom line, find a speaker you like and then find the right amp to drive it.
