I got a multimeter now and measured the following Volt-outputs:
Iriver H120: 3,29V on LINE-IN(plugin-power)
Sony MZ-RH1: 2,39V on MIC-IN(plugin-power)
MM-preamp(with 9V battery): 7,06V on it´s output
SP-SPSB-9 (12V bbox from soundprofessionals): 11,67V (powered with a 12V miniature battery)
I can confirm that the iriver h3x0 has similar output
Note that the voltage drops to around 1.8V when loaded with a microphone.
Does anyone know what resistor value they use? Maybe could lower it a little...
Edit: for H120 that is