Stax headphones require high voltages to drive the electrostatic units, so they can't be used with just any headphone amp (or any speaker amp, come to that) as dynamic-headphone amps produce very small voltages. Also Stax headphones don't use the standard stereo jack. Instead they have their own 5-pin and 6-pin plugs. Which I guess means that they almost exclusively have to be used with Stax amps. Though I did hear of a 3rd party amp that can connect to and drive Stax headphones, but the name of it eludes me.