Thanks for sharing the details. Do you know how many volts this outputs?
As shown by the values used during testing, a 3.1V RMS drive level from the amp is typical for "comfortably loud" listening, with peaks of twice that. The transformers (2 in series) are 50:1 stepup (measured). So 3.1VRMS = 8.7VP-P x 50 = 430V driving the phones.
I often use microwave oven transformers as inductive loads for SE Class A amps. It doubles the voltage swing.
I never use the secondary HV windings - they are left unconnected. They are designed to make 2500v from 120v. I wonder if I tapped into them I could get a nice transformer coupled Class A electrostatic headphone amp! They sure are quiet.
I don't have personal experience with true externally-biased electrostats, but from what I've seen/heard they use HV amps and not transformers. Only the electrets seem to use xfmrs.