Very strange.
Next step: Try another audiomaster. Maybe your dealer can give you a BS4, BC2 or another BS9000 with sw 3.4 for some tests. The audiomaster must be able to handle N.MUSIC. If this will give the same result I would integrate a videomaster to the system, because the BM5 is an audiosource (N.MUSIC, N.RADIO) and a video source (PC - PHOTO, WEB). Maybe your BM5 is "missing" the videomaster (?)....just an idea, because afaik you are the first one with a BS5 without a BV7/9/4 and with a (second) audiomaster.
In any case the first thing I would do is to delete Xanadu from the HD..................
Stefan