Zanon,
I (or you) would have to ask Sean Casey about specifics on his frequency response/impedance graph. I haven't even discussed it. Perhaps the data informing the graph was normalized. Of course, to me, Soul *does* sound smoother than Essence, the "least Zu" speaker so far. But I've said straightforwardly I've considered Essence inferior to Druid 4-08 since Essence launched. I certainly never for a second considered giving up my Druid 4-08s for Essence, but I will replace them with Soul Superfly. But I understand why Essence appeals to a broader population.
There's a difference between ragged and wrong. I don't recall the Stereophile data graph on Essence (though I do remember something like JA conceding that it sounded to him as Art Dudley described, regardless of how it measured) but ragged response that shows short bandwidth abberations against a backdrop of general octave-to-octave balance can sound just fine and convincing, as opposed to a speaker with smoother curve but clearly visible octave-to-octave dysfunction, which can sound completely wrong.
At the end of the day, Zu's graph isn't actionable to me one way or another, but I've had the opportunity to both follow the speaker's development and hear at length in final form. It *sounds* about as smooth as Zu represents, but in a real customer setting it would never measure that way.
Phil