You could measure the impedance yourself, it only requires the following as a minimum:
- generate a test signal (sine wave) with any suitable PC software, and copy it to the GS1 in a supported file format
- a test load, ideally a resistor, but even a headphone or IEM can be sufficient if it has a known resistive impedance at a frequency
- a splitter to be able to measure the voltage on the load, one with 1 female and 2 male 1/8" TRS connectors should be suitable
- something to measure the ratio of AC voltages with - a cheap digital multimeter, or even the line input of a PC sound card (the HD audio codec on the motherboard is sufficient) with software to analyze RMS levels
Measure the voltage with and without the load (if it is an IEM or sensitive headphone, make sure it can handle the level used in the test), and then use the following formula to calculate the output impedance:
Z_out = ((V_unloaded / V_load) - 1) * Z_load
Depending on your IEMs, a high output impedance may or may not be a problem. Some have fairly "flat" impedance and will not sound much different, while the effects on multi-driver balanced armature IEMs can be significant. You can check if your model(s) are measured at
InnerFidelity, and how much their impedance varies over the audio band.