Speakers that open up at low volume levels

Jeff---The ET LFT-8, an excellent loudspeaker with a for-some-reason very low profile. You're right, it's rather inefficient, yet imo sounds better at low volume (as well as high!) than Magneplanar's 1.7, a very similiar low-efficiency planar. A lot of Electrostatic speakers sound great at low volume as well, perhaps as a result of their unmatched transparency.

Thiel, this is why I made it my reference.

Sky isn't limit in this case and there will be trade-offs and trade-off may be the dynamic range. I'm absolute freak on dynamic range and the larger one gets my choice.

They weren't my "cup of tea", but you might want to audition a pair of Harbeth's, if you like that "English sound".  If not, I'd recommend auditioning a pair from the Totem Element line as well.  Both are very smooth, very musical, great sound stage, don't need to be driven "hard" to sound their best, didn't seem to be picky about the electronics driving them, or the room they find themselves in.