I was not a fan of the AD2Ks at first listen, because of my gear at the time and preferring a darker sound. That all changed when I heard them at a meet. The W5Ks are currently my favorite dynamic cans, and I only chose them over the AD2Ks because I needed closed. And those two share enough characteristics that I couldn't justify owning both.
I tend to be pretty accepting of a phones sound if I feel there's balance within its sound. That probably sounds ridiculous to someone that disagrees with on specific cans (say the Sonys or Grados), but I don't think the AD2K/W5K and HD580/6x0 share a lot in common and I like them both. My personal sonic tastes have changed over time and I think like many here synergy plays a big part. I think ATs can sound bright and certainly treble focused losing the low end, cold and even a bit analytical (though with the much repeated flavor that keeps them from ever being unexciting). That can change with the right fit and amp (and especially tube) combination.
You mentioned liquid midrange was important to you and that the ATs barely squeaked by there. I can understand that. Then again I heard someones (forgot members name) Ampex (I think) reel to reel/Zana Deux/AD2K setup and I doubt anyone would fault that there. As for the STAX, is it possible to have a textured, liquid midrange? If so maybe it's here.
There's been a little time since I posted last. It may be that I'm still in my electrostatic honeymoon, but they're getting almost all my time. I have a srm-007t amp on loan and it may finish off my last reservations of hard-hitting music with STAX (which I had with srm-1/mk2 with my phones or the Omega IIs with lower STAX amps).
I wonder if I didn't get some of my gear at decent prices used, if I'd just go with srm-007t-Lambda Pros for everything. I don't have enough experience with other STAX (or other manufacturers electrostatic) gear to comment larger though.
And I have to warn all those considering (especially with recent surge of electrostatic threads/commentary) that these may be too polite for some. Sennheisers, for instance, can have great impact with some gear, but to many they will never be as exciting as Grados. Electrostatics may never be dynamics.
For me, with audio gear (and to a certain extent recordings), I'll always choose hearing something over feeling it. This is a common conversation and hits genre choice, live performance preferences, etc. If dynamics generally are a rock concert, electrostatics (as far as I can tell) are a jazz club. Everything sounds so right and within it's world it's quite beautiful.