SOLD a STAX SRD-X portable amplifier for electrostatic headphones. This is standard bias but can successfully run pro bias headphones. Working perfectly.
This is a portable amplifier unit, running off either a 120v to 12v DC wall-wart (included) or eight C batteries. It's wired to take its input signal via 1/4 inch jack from any headphone amplifier you already have. Note that this not an energizer and does not require an external speaker amplifier for power.
Sound signature is darker than the SRM T1 and actually sounds pretty good. Unit is 100% functional, very good physical condition.
$60 covers shipping in CONUS, buyer to pay any PP fees on that amount.