Koss didn't invent the electrostatic headphone—that was Stax, back in 1960—but they were stubborn enough to try doing it their own way. The ESP-950, introduced in 1983, was their serious swing at the title. By that time, the Stax Lambda dominated the conversation among people who had the money and the patience for electrostatic gear. The Lambda was brilliant and clinical and exact. The ESP-950 was something else: friendlier, rounder, less interested in making you hear every microscopic detail and more interested in making you want to keep listening.
The topology matters here. Koss built their own driver from the ground up instead of licensing Stax's designs, and it shows immediately. Where the Lambda's diaphragm was gossamer-thin (0.25 microns), the ESP-950's was thicker and heavier. That alone changed the entire character. The thicker diaphragm meant higher distortion at the extremes, technically speaking, but in practice it meant the 950 had body. It had a midrange that didn't feel transparent so much as genuinely warm. Bass came up cleaner and more present than you'd expect from a stat, with actual weight to kick drums and cello. The treble was extended but never harsh—a quality that made poorly mastered records tolerable rather than punishing.
The driver required a bigger energizer box than Stax's compact affairs. The ESP-950 came with a brick of a power supply, tube-based in the early revisions, solid-state by the mid-'80s. That thing ran warm and drew real current. It was built like furniture. So was the headphone itself—Koss used a wider, more comfortable headband than their competitors, and the earcups were slightly larger. If you were going to sit with these for three hours, they'd let you do it without your ears filing a formal complaint.
Production ran through the '80s and into the early '90s, with several minor revisions. The original 1983–1985 tube-energizer versions command respect from collectors, but the later solid-state models from the mid-'80s onward are actually more stable and just as engaging. Prices settled into the $600–$800 range when they were new. Now? You'll find them anywhere from $400 to $1,200 depending on condition and whether the energizer still works without smoking.
Here's the honest thing: electrostatic headphones are finicky. They require a proper signal chain. They're sensitive to humidity. The diaphragm can go microphonic if you breathe on them wrong. Parts are getting harder to source. But if you get a good example, one that's been maintained, the ESP-950 will teach you something the Lambda never quite managed—that extreme detail and genuine warmth aren't mutually exclusive. They just require a different philosophy.