Speed kills....

Yes Erik, absolutely. ESL’s are almost often implemented as dipoles (sound launched forward and rearward), but not always as line sources. None of the Quads are line sources, but all are dipoles (though the rear of the 57 comes covered in felt---the center tweeter---and burlap---the entire back of the speaker). And of course a line source can also be made with multiple cones or domes, and as either a direct radiator loudspeaker or a dipole.

Duke,
You work in an interesting way and present what you do quite convincingly. I wish you success.

I don't know why Duke introduced Stax headphones; but that was exactly what I thought when I read the OP.  Is there any way you could audition them, Russ? 

???
2channel8: I have the Lambda headphones with an SRM-1 (mark2, I believe) amplifier.  I'm trying to replicate that sound but on a larger scale.  

When trying to replicate Stax headphone SQ on speakers, don't forget to pay attention to the amp and power circuits. I liked Duke's linking of perceived speed to upper frequency behavior.  I've found improved transient attack to be the biggest benefit to having larger amps and better power cords/conditioners/multiple dedicated circuits. Add to that some treble clarity, and you also get air which is a trait I highly covet. I don't think I've heard horns match hard domes in that area, and definitely not electrostats. Larger speaker cabinets also move less and provide a better launch for the wavefront. YMMV and good luck!