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.

Well, if you are using a transformer box of any shape then a lot depends on what amp you are using to run the transformer box. Unless it's a mighty good amp you already have to use with the transformer box, you'd likely do better getting a stat amp.

Also, transformers are limited in that in transformer boxes they are often built for a low price point and so aren't the best transformers available for the job. If you have a great amp already and want to go with a transformer box, I would look at the boxes Spritzer has available at Mjolnir Audio. In addition to the amps, he has rebuilt several of the STAX transformer boxes with modern components and doesn't compromise on quality.

There are other considerations, but there are other threads on here that answer the question you are asking. I'd suggest searching them out for more information.