speakers a bit closer to price parityFair enough. My non-parity example reflects the dearth of both phase coherent designs and designs with pistonic behavior. Both of these groups are very small and the overlap seems to stop at the Vandersteen 7 (maybe 5 Carbon and out-of-production Thiel CS3.7)! But I stand by my point if we compare the Vandy Quatro CT and Vivid B1, which are at the same-ish price point.
the steep slopes make it easier to get pistonic behavior in the passband....Yes, that’s what I implied earlier ("especially important with XOs using slow roll-off filters "). Even with steep filters you need a well-behaving driver.
I have lived with Thiels or Vandersteens for over 15 years. So, I am very familiar with phase coherent sound. But during that time I’ve listened to many other speakers. My favorites, alphabetically, are Avalon Ascent, TAD Ref 1, Vandersteen 7, and Vivid Giya. Only the 7 is phase coherent, although the Ref 1 has a coincident midrange/tweeter. But one thing these designs have in common is pistonic behavior. IME, that one factor best corresponds to high resolution and transparency.