I have a working Superex PEP-79E Electrostatic Phone. So I have the manual with full schematics and circuit board layout, pin arrangement etc. Mine comes with a PEP-74 headset. The 79E console is a self powered control box using the audio signal to generate the 150 volts bias voltage. It may be very expensive, if not impossible to find the step up transformers.
But it should not be too difficult to modify another brand of bias circuit for the Superex.
The PEP-74 is one of the most natural sounding phone in its era. It uses a light weight, on the ear design to make it comfortable to wear over longer period of time. Once you try this Superex, it will be impossible to go back to those bulky Koss phone of those days. The price of an over the ear phone is moderate sound insolutation of 30 dB.
I am new to this forum and have yet to find my way around in PM or posting the manual file for you. I hope to be able to help you soon.