Pros: Non-fatiguing sound signature, Great Soundstage, Lightweight, Lifetime warranty
Cons: Nagging issues, Perceived build quality, Little "wow" factor, Seal dependent
The black sheep of Electrostatic headphones, the ESP-950 has been around since 1990 and is still in production now, overshadowed by Stax & co, they do not receive a lot of publicity, but are still highly competent in their own right.
Fiio E10 (Line out)
Hifiman HE400i (Grill-modded)
The ESP-950 is an open-backed headphone, and is rather tall, painted all black with mainly plastic parts, except for the metal grill, which gives a sense of flimsiness, on the upside, this construction is lightweight and comfortable on the head.
The stock earpads are pretty meh, but the light weight and low clamping force makes these comfortable on the head nontheless, unfortunately, my pair has developed audible squealing on the right driver, this is apparently very common among ESP-950s, and should be a consideration in buying them.
There are other odd quirks and features with the ESP-950, on the stock E90 energizer, volume controls are separate for each channel, which is great for those with mismatched sensitivity in each ear, the amplifier is also able to be powered by a battery-pack, in case you get the urge to listen to these on the go.
One thing that is AWESOME about the ESP-950s is the lifetime, no-questions-asked warranty that Koss provides, simply ship them the headphones with a note detailing the issues and a $9 check, and they will repair it, big kudos to Koss here.
Overall, the ESP-950s are a mid-focused pair of headphones with an emphasis on being non-fatiguing and smooth. I was not impressed with these at first, as they seemed rather boring in presentation compared to my other cans, but over time, I came to appreciate how forgiving and non-fatiguing these cans are, without sacrificing technicalities.
The Bass is well-extended and reasonably linear* down to the subbass. Punchy with very quick decay, lacking in impact and rumble compared to dynamic and planar counterparts, HE-400i's are more visceral and have more texture, while TH-X00s feel like subwoofers in comparison. Overall, IMO they're not bass light headphones, but bass-heads and those who can't live without impactful bass, look elsewhere.
The meat of the ESP-950 experience. They are a very different type of presentation from normal dynamic or planar headphones, with a sense of lightness and effortlessness that can seem boring at first, but grows on you with its ability to reveal low-level information without being overly analytical or in-your-face. This is helped greatly by the great separation and staging these cans possess, the downside to this sort of ethereal presentation is that the ESP-950 can lack engagement and note weight, which is a mark against it in genres that benefit from a more visceral presentation.
Treble is well-extended, albeit slightly behind the mids, they lend good airiness to instruments. The FR is very even with little peaks, meaning that sibilance is very rare, only noticeable in very hot recordings, and even then is well-controlled and doesn't stick around. HE-400i and TH-X00 are both not able to match this, and are more fatiguing in comparison.
A big strength of the ESP-950 is the soundstage, these cans are up there in terms of their ability to project a big soundstage. The soundstage is very, VERY wide, which allows for great separation, and ability to single out sections of complex passages, which is a key part of their ability to be resolving without being fatiguing, soundstage depth is less impressive, but still good enough to allow for accurate imaging.
The ESP-950s have a very coherent presentation, no part of the FR feels particularly recessed or forward, meaning these headphones play well with many different genres
*Bass significantly rolls off if seal is broken, which is easy to do given the height of the headphones.
TL;DR - A versatile set of cans that can play many different genres, a laid-back, wine-sipping complement to the agressive, shot-chugging TH-X00, the ESP-950 are a great pair of cans if one can find them cheap and/or used, for the price I paid for them, they are a MASSIVE bargain.