A high efficiency speaker is typically made of a horn tweeter and a fairly large woofer cone and most likely a 3-way to smooth out any gaps in the dispersion pattern. I wouldn't call that a "monitor" but tube amp lovers are hoarding things like the Klipsch (heritage speakers like the Heresy) for this very reason.
50 years ago amplifier power was scarce and expensive and speaker manufacturers paid a lot of attention to efficiency. Nowadays amplifier power is both cheap AND efficient (class D) and speaker manufacturers paid attention to size instead. Modern speakers, like those Selah Audio ones Rick make, both sound better and are smaller, the only downside was they were not as efficient as yesterday's speakers, but to solid state amp users this is the least of worries.
Today, TUBE amplifier power is still scarce and expensive, so we may have to look into the approaches taken in the post-war period for inspiration.