Jon,
To recap and make sure I have your "Problem Statement" correctly understood
- Listening to N.Music on through your Beosound 4 and Beolab 6000's gives a crackling noise, but DAB,CD, Radio is fine. Also listen to N.Music over the Beolab 4000's connected to the Beoport works perfectly.
- You have replaced the Masterlink Cable and mains cable.
However, you did not confirm if N.Radio gives the problem in the same way as N.Music ? Please confirm
Assuming nothing else is connected to the Masterlink cable other than the Beosound 4 and Beoport and N.Music AND N.Radio give the same problem, then the problem is either with the PC or you have an earthing issue.