I have a stand alone Koss E90 box and a Stax SRD-7 transformer box. I am just curious which one every one prefers and why? iFi has their eESL trans former box, which is as about as universal as one can get, and there is of course used Stax transformer boxes. There is also a plethora of electrostatic amps, stax, Woo, Mjolner and so on. Just curious as to what every one likes for their Stax or other electrostatic headphones.