Q: If I use the Xone:4D soundcard (for example with Traktor, or just with iTunes) the sound is terrible, practically no low and very sharp sound. 
If I use an external soundcard with Xone:4D hardware I don't have this problem.

A: It sounds like you put the small black RCA phono shorting plugs into the soundcard outputs on the rear of the mixer.

These plugs are intended for short-circuiting the phono-inputs with magnetic cartridges (record decks/turntables) when they are not in use.

If shorting plugs are inserted in the soundcard outputs, this results in very poor audio.