For years, I could walk into a listening room and tell whether it was a panel or a box playing - sight unseen.As much as this "looks bad" there really is no evidence (and there has been a fair amount of research) that this "sounds bad". Intuition can be useful but not always and I think phase and square waves are examples of things where it makes intuitive sense that good square wave reproduction and flat phase should sound better but there is little evidence this is the case and a lot of evidence that it doesn't make any audible difference. This is backed up by my experience with REW and Rephase to flatten phase.... I can measure and see the "success" of the phase correction but if I ABX with and without the phase correction I can not tell a difference. I think this is valuable information because it prevents me from wasting time and money on things that don't matter.
Was it identifying box resonances? Or was it phase - which panels like the Quad ESL63 to current do so well? (or both?)
Whichever it is, "boxes" have seldom sounded "real" - although the best have sounded very good indeed.... souding good, and sounding "real" are two different things. I do wonder whether that ability to reproduce a decent square wave, even though they lacked both bottom and top extension, was the magic of electrostatic speakers?