Always assume your package will be handled in the Jim Carrey "HDS Delivery Man" fashion, and pack accordingly.
Rule-of-thumb:
One set of high quality bookshelf speakers =
Amana double-wide refrigerator box + 500' roll bubble-wrap
Rule-of-thumb:
One set of high quality bookshelf speakers =
Amana double-wide refrigerator box + 500' roll bubble-wrap