RNS-E Sound quality

I retro-fitted an rns-e unit to my A4 a couple of weeks ago, to start with the sound was excellent but now for some reason when playing a cd the sound is very flat, it sounds as though its only going through 1 mid-range speaker ( all speakers and sub are working ). I have played about with the bass, treble, balance + fader controls but still no joy. When using the radio the sound is great. I have tried different cd's. Any help would be appreciated, cheers guys.