Different drivers as well as different architectures will play at low levels differently. One of the reasons I’m a horn-guy is because of the remarkable (in general) resolution at low listening levels. The fact that they can go out to "ffff" as well is an added bonus.I particularly like horns because they can sound dynamic, lively, and satisfying at quite low volume levels. It is just a guess, but, I bet that is part of the reason they are so popular in Japan where everyone lives close together and no one wants to offend or inconvenience their neighbors.
I think there are quite a few panel speakers that sound VERY realistic and are capable of playing at respectably high volume levels. I would never categorically exclude them from ANY list for any particular performance parameter, except perhaps, compatibility with very low-powered tube amps (Quad 54 fans might disagree on this exception). Can soundlab U-1s rock, or Sanders Sound Model 10 or almost any Maggie? You bet they can.
Many electrostats can do the low level thing as well. This isn’t about panels however. The older Magnepans needed a solid kick in the rear in order to come to life. They sounded a bit dull at low levels. I haven’t heard their recent efforts.
Low level listening (and whether it’s important to you) is frequently overlooked, and in no way am I diminishing the importance of playing at realistic levels, but let's be honest with ourselves - how often are you going to play The Who's "Live at Leeds" at "correct" SPL's?
Cheers,
Thom @ Galibier Design