+1 about removing the tubes and blocking the transformers. I've had
many more problems with speakers than amps. Here are some EV SRO's I got a while back;
Two of the 4 were in this condition, no obvious damage on the outside of the box. In the last few years I had 2 SRO's, 3 K140's, 2 K145's, a D140F, and a Celestion (
in a Trace Elliot bass cab) destroyed in shipping. All packed well, all suffered traumatic life ending strikes generating enough G force to shatter cast frames, shift magnets, rip cones, etc. As far as shipping tips for drivers, #1 would be to not ship them at all if it's possible; go get 'em. Since that rarely is an option I'd say in a speaker cab is best, followed by bolted to a piece of plywood with
plenty of cushioning
under the plywood and behind the magnet as well; those EV's were bolted together face to face with about 4" of styrofoam behind each magnet, in a tight fitting box.
My guess is the box was dropped flat onto a slab from about 5-6 ft. or so. The magnet on one of them flattened the 4" of foam into a half inch thick piece of masonite. Not many speakers will withstand
that kind of impact regardless of how well they are packed.