You really want high end and ultimate efficiency? The James B Lansing corner horns of the late 50's can't be beat. The ones that have a chance for stereo placement in typical rooms are the C-34 "Hartsfields".....rear-loaded horns that do alright away from the corners (although corners are better).
high-efficiency loudspeakers
What is the best high-efficiancy loudspeakers? If you have it, are you happy ?