The STAX SR-009 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.
STAX publishes 101 dB / input 100Vr.m.s. / 1kHz (100 Vrms input at 1 kHz — a voltage reference, not a milliwatt one). 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 145 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: stax-international.com.
STAX prints no frequency range for the STAX SR-009 at all. Source: stax-international.com.
Stax keeps a specification page for the SR-009S, the model that replaced this one, and prints 5 – 42,000 Hz there. For the SR-009 itself there is no longer a page of its own.
That is less unusual than it sounds, and arguably more honest than the alternative: of the 578 models checked so far, 490 carry a maker-printed range, and only 58 of those print the ±dB tolerance that would make the range mean anything.
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.
145000Ω · — · Electrostatic
See requirements →145000Ω · — · Electrostatic
See requirements →145000Ω · — · Electrostatic
See requirements →60Ω · — · 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 STAX headphone in the database, and how much of what this site says about them came from STAX’s own documents.