Inside the Stax SRD unit, there are a pair of step up transformers for audio signal and a step up voltage circuit for the bias voltage.
The audio signal needs to be amplified a lot for it to be played on the electrostatic headphones. Many of electrostatic amps can swing the audio signal to over +/- 400V. The bias voltage for electrostatic headphones also needs to be stepped up from the main AC voltage. For Stax headphones, the bias voltage is 580V. So, the amplified audio signal together with this bias voltage of 580V can actually be more than 1KV.