I would put some styrofoam in between the wood from the box and the wood that will take a beating. Especially around where the subs are. Make you sure you write TOP where the subs are facing, so hopefully someone will b more careful around it.
But to be honest I work at fed ex, and when people get huge boxes like that, too often they drop them off the belt cuz their heavy. Ne way you ship that, its going to get banged to death so make it sturdy. Ship it using fed ex freight, and you might have a better chance of getting it there in good condition.
just my 2 cents.