Complex sentence: The library of the cathedral in Antwerp was destroyed by religious fanatics in 1556, probably destroying most of Barbireau's music, but some has survived, mostly in foreign sources, and that which has survived is of outstanding quality.
Split into simpler sentences: The library of the cathedral in Antwerp was destroyed by religious fanatics in 1556, probably destroying most of Barbireau's music. Some, however, has survived, mostly in foreign sources, and that which has survived is of outstanding quality.