Welcome to the DIY world.
1. For typical tube amps, the amps are coupled with output transformers, and therefore the output voltage swing isn't all that high. However, the amps for electrostatic headphones are connected directly to the headphones. The voltage swing is as much as the power supply.
2. Voltage swing from an amp can indeed be lethal. So, we have to make sure that the headphones are safe enough to wear. We normally use single sided...