It definitely is the sellers responsibility to pack and ship the item properly so it arrives intact and fully functional. If the package was obviously damaged by the shipping company, that is one thing. However, the box wasn't torn to shreds, so the item was damaged by poor packaging.
Even if the sub was NOT removed from the enclosure, it should not have broken the enclosure. Going over a large pothole or major bump would do about as much jolting as most shipping. I ship a lot of electronics (some VERY heavy), coast to coast, and have never had a problem....in over 12 years of shipping.