The Koss ESP/95X Plus is an electrostatic headphone. It does not connect to a normal headphone output at all — it needs a dedicated energiser that supplies a high bias voltage, so every power figure on the rest of this site is irrelevant to it. No dongle, amplifier or player will drive it, whatever its wattage.
That is the whole answer, and it is worth stating plainly because "how many milliwatts do I need" is the wrong question here. What you need is a matching energiser from the same ecosystem.
Electrostatic specifications are quoted differently and are not comparable to dynamic or planar headphones. They are listed here only so the database is complete.
Koss publishes 98 dB SPL/1mW (Koss states no frequency and no other measurement condition beyond the unit). That is a sound pressure at a drive voltage of around a hundred volts on the energiser input, which is how electrostatics are specified — there is no milliwatt figure to convert it to, because nothing about this headphone works in milliwatts.
The 100 000Ω on this page is not a resistance either: an electrostatic presents a capacitive load, and that figure is an impedance quoted at one frequency. It is listed for completeness, not for calculation.
Source: koss.com.
| As it is labelled | As printed | Tolerance | Standard |
|---|---|---|---|
| Frequency Response | 8-35,000 Hz | not printed | not printed |
Koss prints 8-35,000 Hz for the Koss ESP/95X Plus, under the heading “Frequency Response”. Source: koss.com.
No tolerance is printed with it, and that is the whole problem. “8-35,000 Hz” says nothing about how loud this headphone actually is at 8 Hz: it could be thirty decibels down there and the sentence would still be true. A range only means something with a ±dB figure beside it, and of the 490 models whose maker prints a range, 58 print that figure.
What these numbers are worth, maker by maker, is set out in the frequency response audit. No measurement is published here — only what the maker printed.
60Ω · — · Moderate
See requirements →200Ω · — · Moderate
See requirements →200Ω · — · Moderate
See requirements →200Ω · — · Moderate
See requirements →Or open the comparison tool to put any two models side by side, or browse all 578. Or see every Koss headphone in the database, and how much of what this site says about them came from Koss’s own documents.