Fronts: Acoustat 2+2's directly driven from Acoustat Servo-Charge Tube amps.
Center: None
Surrounds: Acoustat 1+1's with MK-121C Medallion interfaces.
Subwoofer: Outlaw LFM-1

I sit nearly 20' from the front so the absence of a center channel is not a serious liability. Since the system fires down the long axis of the room, even when seated off center the dialog channel doesn't shift off the screen too much. Also, integrating a center channel speaker with electrostatics can be problematic. With that said, I would still like to experiment with a center channel speaker. I have been accumulating some salvaged Acoustat parts and components with the hopes of putting a DIY project together. A rear center surround is something I would like to try too.