Does anyone of you have noticed a stereo shift on the headphone output of the RME fireface 800?

I've hard wired the stereo headphone output to an external recorder, played a mono signal (440hz) and recorded it. After analysis both channels are not at same level... Left -13,9 Right -13.5...!!?? Furthermore at very low level the stereo shifts completly to the right...

Another user has the same problem. Go here.

Config: Win7, last driver...