I have received the replacement for the bent headband of the Koss ESP900 and also finished building the Stax2Koss adapter, so it's time for some impressions.
First the adapter: I don't have access to a 3D printer and any friends who can machine plastic, so I decided to try to use hot-melt glue. The result is not really nice, more like a hot-melt glue mess. But with the heat shrink tube attached the mess is perfectly hidden and the adapter works like a charm. Let's hope it will survive the next summer.
To the headphones: The first impression points to the ear cushions, they are made of a hard plastic with a smooth surface. So they are not very comfortable and you sweat easily under them. I assume easy cleaning and disinfection was a requirement for the medical use. I assume that the cushions of the ESP950 will fit, so I probably will buy a pair as an upgrade (When doing so I can maybe open the cans as DefQon requested ).
Second impression: I know now what the "Koss squeal" is. It's not permanent, it was very annoying when I first tested the adapter before I closed it with the heat shrink tube, but it disappeared after that. During todays session they were quiet at first, but the squeal came back after some minutes of listening. Since it already happens with CD off and volume set to zero it only seems to be related to the bias and possibly some static charge.
Listening session: So today I quickly arranged some stuff in the kitchen for a small comparison session, involved were a Cambridge Audio CD player, the Stax SRM323 amp, a Violectric V200 amp, a Stax 404LE as electrostatic headphone, a Beyerdynamic DT770 (600 Ohm) as closed headphone and of course the Koss ESP900.
When I knew how the result would be I wouldn't have moved all the stuff, since the differences are anything but subtle.
The Koss has a very thin sound with nearly no noticeable bass. It may work for some vocal music but a CD like Dire Straits' Brothers in Arms is a pain to hear with the Koss. In the mids they have the speed (don't know better how I should call it in English) why we love the electrostats and in this and only this point they outperform the Beyers. There is nothing to compare with the 404LE, worlds are between it and the ESP900.
The damping of the closed construction is not very high, much less than the Beyers even less than the Ultrasone HFI-780. But at least the damping seems to be similar over the frequency range which is an advantage over the HFIs which seem to favor some frequencies. Possibly some softer ear cushions could improve the damping since the hard cushions don't seal very well (I should say that i have a small head, so the pressure with which the cans are pressed to the ears is also small).
So as a short summary I wouldn't really recommend them. I expected to have some inferior sound compared to the Stax but hoped I could use them due to their closed construction when (mostly TV-) noise is present in the same room, but in this case I will have to continue using dynamic cans.