I had this Banham after installing the Kufatec MDI interface lead.
What it ended up being was at the back of the Kufatec quadlock, the speaker wires were loose (having used for it over a year, I'm not a huge fan of the quality of the Kufatec lead) and ended up in a very distorted crap quality sound from 2 of the speakers. CD and radio were unaffected, so sounds very similar to your problem.
I ended up wiggly those wires (I forget the colours off hand) and it improved/changed the quality depending on how they sat.
Check this firstly and I'd even have the unit out while playing music down the MDI while wiggling wires in the quadlock. You may find you aggravate one that changes the sound then that would appear to be where your problem lies