http://apuresound.com/ehrc.html has adaptors and will also change the Koss cable on your ESP-950 for a Stax cable/
I've got ESP-950's with a Stax cable on them, and I have a Stax-to-Koss adaptor so I can listen to my Stax or my Koss headphones on the Koss E-90 amp OR on Stax amps.
I had an SRM-1 mk II but I sold it. I have an SRM-T1 that I like better.
The Stax amps sound better on the ESP-950's than the E-90, but not WAY better. 10% better? 15%? 7%? Better, but not much better.
You might consider just saving your money and stay with the E-90. Yeah it's made of plastic, but actually it's a well designed amp.
Of course, if you have a supply of spare cash, and just want to try it out- then go for it.