I figure I'll post this here and on the Kaldas RR1 thread. This weekend, for fun I made a Stax to Koss adapter, so that I could use my Stax terminated headphones with the Koss E90 amp. I've attached the picture below.
A friend had used the female end of my Koss extension to make me a Koss-to-Stax adapter. I saved the male end, and this adapter was a lot easier to make. I used Spritzer's advice and added a 4M7 resistor to the bias line - but otherwise, the female socket I was able to find online was much easier to solder to than the common male plugs. Without the resistor, I believe the problem is that Stax headphones are more likely to arc, thereby damaging your diaphragms.
The Stax cable I bought from Spritzer for the Kaldas RR1 fit flush as shown in the image. The pins on my Kaldas RR1 cable are longer and end up being exposed. In any case, both cables work just fine. I may add a cylinder or backshell to make sure pins aren't exposed in any case.
This might be useful for people wanting to dip their toes in the Stax or Kaldas headphones, without having to invest in an amp right away. The E90 did a decent job of driving my Kaldas RR1. The signature was brighter than I expected, and I used a tone generator to see if the E90 gave me more extension on the high end. It did not - I could hear a slightly lower max frequency with the E90 than with my SRM 717 (14.5KHz vs 15KHz). But the overall tonality was brighter, driving the Kaldas closer to neutral. The bass was still great, far better than my Koss ESP95X - but nowhere near as good as with my Stax SRM 717 amp.
The Stax SRM 717 provides better soundstage, a little more clarity, and deeper bass. Would I use the E90 as my primary amp? No. Would I use it at work as a secondary amp? Absolutely, if I'm eventually allowed to return to the office after this crisis.