Vandersteen Model 2D?

pistonic not much good IF ya throw it all away with a filter that screws up phase because non pistonic is out of phase behavior...they are same thing, one is mechanical.
now having said that, obviously w very worthy goal and a reason why the CF/ Balsa drivers are a wonder.... I can tell you they are electrostatic like in purity...

but yes they are expensive....the balsa must be top grade, indeed aerospace grade, ditto the CF, CNC machining required for tapered cone - optimized for mass, stiffness, and as others aptly pointed out breakup modes far away from filter set points. Autoclave required for cobonding and oh ya, cobond to end grade balsa is a trade secret, nobody else doing it..because it is hard and requires years of R&D = ¥€¥ or $...then all that needs to be integrated into an aerodynamic ( patent expired ) driver. Think this thru you go to all the trouble and expense to design/ build a pistonic driver only to let an out of phase reflected back wave go right thru the cone....

but yes, some of us stay up late thinking about how to build that driver for less money...........
Kaizen is life
or so my sensei says, he would know......Chihiro Nakao 

non pistonic is out of phase behavior...they are same thing

I don't find the sound of a driver at break up mode the same as a speaker with a high-order filter, if that's what you're meaning. One is distortion and the other is a very subtle sonic signature. I would much rather have something like a Vivid B1 (pistonic but with out-of-phase filtering) than a Vandersteen 3 (phase coherent but not as resolved because of the non-pistonic diaphragms). You can "have it all" with the Vandersteen 7 but that product is just a bit out of my budget :)

Beetle....maybe....but perhaps we might compare speakers a bit closer to price parity 3:1
is a stretch even for the frugal Dutchman to overcome...

but yes, i would say the KEF and B& W alumni who believed in FFT and laser inferometry of drivers would have a bent for that..

the steep slopes make it easier to get pistonic behavior in the passband....

i am spoiled, admit it.....

search utube and you can find some cool video of Vandersteen 7 drivers and another brand X undergoing the lastest laser measuring technique...out of phase in RED, sometimes 10% of output... 

and yes I was being dramatic, I think understanding the trade ratio of pistonic vs phase and time accuracy might be interesting, especially as measured vs what we actually hear...

speakers a bit closer to price parity

Fair 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.