The model "One Pi" from Pi speakers. It seems to be pretty sensitive at an honest 94db/m with a bening impedance curve for an SET amp. At 12"x18", it's not a mini-monitor, but likely as small as you're going to find, while being fairly efficient with decent low end extension.
http://www.pispeakers.com/Products.html
http://www.pispeakers.com/Products.html