As a relative novice to electrostatic headphones, I've been trying to determine what parameters are best for a particular headphone. Lambda Pro in ES1 is superb, but I discovered that my tube complement was restricting the voltage swing a bit. Juicing up the amp brought the HE90, and a visiting Omega II, to life, but proved to be a bit much for the Lambda Pro, which is now happier being driven by SRM-007t (sound got sloppy with the huge voltage swings the other cans demanded, and it was clearly being overdriven). I've heard the OII sound good out of 007t, but sometimes the warm-up time was unreal, and it often sounded lackadaisical. The ES1 can grab the OII by the cojones and pull out everything it has. That headphone likes its volts (and ES1 can provide a lot more than 007t).
So, if my limited experience is a guide, somewhere in the past 20 years Stax may have beefed up the voltage handling capability (and possibly needs) of their headphones. I'm refering to the audio signal, not the bias voltage. Can anyone confirm or refute this?