There was one active system I was very interested in, but then the designer/builder Roger Modjeski (of Music Reference of course) died, and the system with him.
The system consisted of an electrostatic loudspeaker with no input transformer (all other ESL's have one), the ESL screens driver by a dedicated OTL amplifier. Get it?; a tube amp with no output transformer, and an ESL loudspeaker with no input transformer, the tubes themselves directly driving the ESL diaphragm. I can only imagine the transparency!
The panels had a 4th-order high pass filter at 100Hz, and an 8" woofer in a sealed enclosure, with a 4th-order low pass filter at 100Hz.