Matchstikman--to answer your questions to me: (1) there are simple formulations as to efficiency. If I remember correctly, and I'm a listener, not an engineer, speaker efficiency is measured in decibels of output per watt of power input. Speakers rated under 90 db. are considered more or less inefficient, over 90 more or less efficient. But it's more complex than that: my speakers were just upgraded, and while their output rating only increased by 2 db., the manufacturer's power recommendation went from 200 watts per channel minimum to no upper limit to 30 watts minimum, 300 maximum. Inefficient speakers driven with inadequate amps tend to sound opaque and closed in. The inevitable tendency is to turn up the volume at the preamp, which only makes the problem worse, because the power amp is now working even further outside its comfort zone. My own experience is when I have added a more powerful amp or switched to more efficient speakers, I wind up listening at a lower volume because more detail reaches me, both on quiet passages and loud ones. (2) I listen at a volume I can get away with in a New York apartment, which isn't all that loud. (In fact, I use a 4-channel system, because the rear speakers increase the sense that the sound is filling the room without my having to blast the overall level.) (3) 22 X 13.5 X 9.
