http://www.stereophile.com/content/koss-esp950-electrostatic-stereophones-page-2
Problems
I encountered a number of functional problems with the ESP/950. Firstand perhaps more ergonomic than functionalthe design and mounting of the RCA jacks on the rear panel of the E/90 energizer/amplifier make it very difficult to insert audiophile- grade plugs. The jacks themselves are mounted to an internal circuit board and protrude through holes in the rear panel which are not much larger in diameter than the plug itself, making it impossible to fully insert plugs with a wider cross-section than your typical Radio Shack connector. I used a 1m length of Music Metre Silver for most of my listening tests. It worked, as did AudioQuest, Cardas, and Monster's top connectors, but with a less secure fit than I would have liked. The do-it-yourself audiophile could remedy this situation by enlarging the holes in the plastic rear panel (the latter removes easily). I have no idea how this might affect your warranty, but a spare rear panel from Kosswhich would return the E/90 to an as-new condition couldn't cost more than a few bucks. But you shouldn't have to do this; the use of high-quality plugs should be assumed in a product such as this.
But the plug problem was only a minor irritation. There were two other problems with my first sample of the ESP/950: occasional very low-level noises, apparently originating after the volume control in the energizer/amplifier, and mild electrical shocks to my outer ear from the right headset. The latter was infrequent and not alarmingjust a small pinprick on the top of my earbut it was clearly not normal behavior. I asked for a replacement.
The second sample displayed neither of these problems, but threw in a new one of its own. Periodically, usually coinciding with a head movement, both earpieces would emit a peculiar crackling noise which I can only describe as a short, abrupt rainshower. It was not frequent enough to interfere with my enjoyment of the 'phones, and was usually over before anyone even got damp, but it continued intermittently throughout the evaluation period.
Finally, just as I was wrapping up the review, a new intermittent problem reared its head: a right-channel hum clearly coming from the E/90 energizer/amplifier following the volume control (which had no effect on the level of the hum). This hum also modulated the audio signal in that channel, which took on a warbly, underwater quality. I was able to correct the problem by shutting off the E/90 for an hour or two, after which it would work fine for a couple of hours, when the problem would recur. Koss Fed-Ex'd me a replacement E/90. As I write this, I have had it running over a weekend for 60 hours continuously with no recurrence of the problem. I will continue listening to it and report at a later date if the problem returns, but as of now the defect seems to have been specific to the first E/90.
In a product of lesser sonic accomplishment, these would be matters of significant concern. Koss does, however, advertise a No Questions Asked Limited Lifetime Warranty. In words right out of the ESP/950 manual, "if anything ever happens to any part of your ESP/950 System, Koss will repair or replace that part, no questions asked." (my emphasis)
