According to their website, the S2 has a transformer in the output, so things should be okay with low impedance phones.
Also, the site says
1/4" stereo TRS output has selectable gain attenuation between 0dB and -6dB for greater volume control range on higher sensitivity headphones
1/4" stereo TRS outut on -6dB gain attenuation setting has selectable output impedance between 11 ohm (Low setting) and 100 ohm (IEC setting)
Edited by clasam - 11/30/12 at 11:32am