I think it's got to be panels, they have better detail and soundstage than cones at low volume. May be because the moving membranes are lighter than cones, being much thinner and not requiring to be rigid.
There is no evidence of this I am aware of. That would show up in improved distortion at lower volume which I do not think that is the case.
I expect primarily this is a factor of a higher ratio of direct sound versus indirect sound with panels.