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.
If you have lost your gold phono shorting plugs (part number - AL4396), more can be ordered from your local A&H distributor