Koss makes high-end electrostatic models. The current model is the ESP-950, which retails for $500-1000 or so. I am still trying to find more info about it, reviews are rare, save for a few very glowing reviews on some not-so-reliable sites (like audioreview). It would be interesting to hear how they compare to Stax and Sennheiser models
The Pro4aa isn't high-end by any means. Maybe in price only. The current iteration of the Pro3aa is terrible - hopefully pro4aa is better, but if it's a refined evolution from the pro3aa, it has some very poor base materials to work with. I haven't heard it though, so I won't say.
I've heard mixed things on the A-250, and haven't heard it either, so no comments here too.