Paul,
I checked. I have the SRD-6SB (self biasing) version.
From their description:
Different from the low-impedance type conventional dynamic headphone driven directly by the amplifier output, STAX electrostatic earspeakers are high impedance type and they call for high voltage to drive. The function of the SRD-series energizing adapters is both to transform the amplifier output to this requirement of the earspeakers and to provide the polarizing voltage to the diaphragm....
(more in their PDF)
There are also big differences in the earspeaker models themselves. I like the Sigmas, but I have not seen them available, nor have I heard the newer models.
Bill