Recently, at the Capital Audiofest, outside of Washington DC, I heard some remarkable, very high efficiency speakers made by Charney Audio. The system I heard cost $17,000, but, they do make cheaper as well as more expensive systems. They, like many high efficiency systems, sound particularly good at lower volume levels and would be very good in a small room (the room at the show was small). I liked the Charney speakers I heard at the show enough to be considering them even though they cost a fraction of what I currently run.
I have also heard, and liked, Rethm speakers and the Tekton speakers that are also reasonably efficient.
Do you have any hum or other noise issues with the M-60? Every one of them that I heard had a very slight hum. This hum would be more pronounced with high efficiency speakers, particularly if you will be sitting close to the speakers in a small room. That would be another issue to consider when going with higher efficiency.