My 2c its actually with the arrival of DSP and very good coaxial/coincident drivers we can explore if perfect step response matters .
My KEF LS60 has "phase compensation" as a feature .
Before that, with very shallow passive filters and separated drivers the enormous overlap made all kind if interference problems in crossover region. The off axis response was probably the victim here , you could probably optimize direct sound for a given narrow listen position . MOving the burden to the end custommer to make extensive acoustic mods to the room .
And it made huge demands on the drivers , so power handling distorsion etc would suffer . Expensive Dynaudio tweeters was probably developed for this application ?
Cone breakup in bass midrange made all kind of notch filters necessary etc etc .
My point is that the general design demands of a passive speaker to really be of first order type with perfect step response would make it very different in many other parameters .
* many drawbacks that would make it a worse speaker than otherwise possible . If you "give up" this first order feature you gain massive benefits in other areas.
You must really have an unlimited budget making it a multiway like dunlavy or something similar ? otherwise the best compromise seems to ditch the first order xover to gain other benefits in any normal budget . So its was a niche
* one could not possible separate out the "phase time cohernce" factor from other factors when listening as the design has to be so different to make this possible.
Does it sound different due to perfect step response or because the whole design has to be optimised for this ?
I've owned Thiel speakers they sounded beautiful , so i have noting against this kind of speakers , but i remain unconvinced that's its a good compromise ?
You seems to gain very little or possibly nothing ? Very few recordings and rooms makes it even possible to appreciate the difference if it indeed is audible ( research indicates its not )
But with arrival of active DSP speakers you can have your cake and eat it

in that context its very well worth exploring , you dont make damaging compromises or make the product more expensive .