Must speakers have certain volume to sound convincing ?

Panels sound nothing like real music, as beautiful as they can do. They are for dreamers not for those who want to hear it as it is.

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.