volume/transparency ?

Yep, it is our ears more than the speaker. The basic premise of Fletcher Munson is that our ears are far less sensitive to low frequencies and high frequencies at lower volumes. Our ears are more or less midrange dominant at lower volumes. So, we don't "hear" the full range sound until we start to turn it up closer to the original level it was played at.

Streakster -

I find a correlation between high efficiency and low-volume liveliness in conventional speakers, and a correlation between transformer quality and low-volume liveliness with electrostatics (although a very thin diaphragm also seems to help). In general, I find electrostats to have more liveliness and inner detail at low volume levels than conventional speakers. The most lively speakers at low volume levels I've heard are full-range electrostats and full-range horns.

Compression is largely a function of voice coil heating with conventional speakers, so a high effiency speaker that undergoes little voice coil heating will compress less. Sometimes in a multi-way speaker the various drivers will have differing efficiencies, and therefore differing compression characteristics. In such a case, the speaker will sound "right" at a particular volume level, but will sound tonally unbalanced at higher or lower volume levels.

Transformer losses are the primary cause of compression in an electrostat, with transformer saturation resulting in severe compression.

To see a set of Fletcher-Munson curves, go to this address:

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eqloud.html#c1

Steakster, I don't quite understand your question, so I probably haven't answered it. Is it the imaging that collapses at lower volume levels, or the clarity (transparency)? What speakers do you have?

Audiokinesis, nice post. But re differing driver efficiencies and compression limits: why are these related?
The higher sensitivity driver (almost always the tweeter) is padded anyway, so compression issues would be unrelated to efficiencies, no? Not relevant here anyway, but just a thought. I would think that speakers that have a "right" volume only are (accidentally?)designed to produce an inroom response that flattens out VIS-A-VIS our fletcher-munsoned ears. It's an interesting point. In addition to symmetrical "droops" with lowered SPLs, I wonder if the best designers also have to factor in off-axis flare smmothness AT DIFFERENT SPLs to get a speaker that sounds balanced in a wide range of volumes? Interesting....