That is an odd sentiment that there's a lot of current required to drive an electrostatic headphone at low frequencies. They are a capacitive load, and the SR-007 will load an amp at 20Hz about as much as an 82 million ohm resistor. I'm having trouble counting all the zeroes, but I think that's 7 micro amps of peak current at 600V peak to peak drive.
I do agree that only having 3mA of operating current is not nearly enough for any electrostatic headphone, and would indeed give a thin, rolled off sound. I run 10mA of current in the output stage per channel, which strikes a balance between keeping the plate load resistors reasonably high, overall dissipation in the output tubes and loading resistors reasonable, but also extracting solid performance.
