I recently built a Koss to Stax adaptor to allow the 950 to continue to be run off its own amp but at the same time to allow it to be run off Stax amps using the adaptor.
My plan is to run the 950 off one of my older Stax amps, a transistor SRM3, which must be about 10-12 years old and has since been replaced by 2 comparable newer transistor designs as well as by several tube models.
Having tried the 950 on the SRM3, I wanted to compare the E90 directly. The Stax amp has a parallel set of outputs which allow you to hook up a speaker amp, or in this case, another electrostatic headphone amp.
The Stax amp was tweaked with sorbethane footers and an upgraded, no-name power cord(made by a local dealer) The Koss 950 had a sorbethane sheet underneath(it was too light for footers) and its wall-wart 12 volt power supply was replaced with a 3amp Elenco bench power supply. Previously I had noted that the Koss gained a great deal from a beefier power supply and the Stax amps needed the help of an upgraded power cord.
I then compared the two amps running the 950 phones and listening to some rock/pop and classical (Elvis Presley, Springsteen, Pink Floyd, Mozart's Requiem and Prokofiev's Suite from Love for Three Oranges.) I used a Sherwood Newcastle cd placer as a source. This is a cheap but decent machine.
The most obvious initial difference was the wider and more stable soundstage of the Stax amp over the Koss amp. The Koss sounded like it had slightly more bass, probably about 100 Hz, but the bass was somewhat congested sounding. Deep bass seemed about the same with both and it seemed as if the treble was slightly rolled off on the Koss. The dynamics and punchiness seemed about the same with both amps, possibly the bass peak of the Koss gave it a little edge here, but at the cost of some transparency of sound.
I definitely preferred the Stax amp, but the Koss amp is not junk either. If the Stax was scored at 100, I would rate the Koss at about 85.