Are pallets mandatory for shipping crates?

just tried to paint a picture in my mind's eye of your situation - it appears to me that you should have pallets. The speaker boxes could be strapped to the pallet thereby making the whole shipment very secure.
The shipping company is very likely going to have just 1 person deal w/ your shipping. He is not going to personally carry two separate 100lbs boxes. They are probably going to use a forklift to move these boxes around & if you do not have a pallet the forklift blades are most likely going to pierce the boxes thereby ruining your speakers + getting the next buyer really mad!
Get the pallet - it'll give sufficient clearance from the ground to allow the shipping co. to use a forklift w/ minimal damage. IMHO. FWIW.

PUT THEM ON A PALLET! If you do them individually they very well might topple them end over end to move them around. If you don't think they can handle a drop from standing to laying down then absolutely palletize them. I've never had a problem once with a shipment on a pallet, I can't say the same for individual speaker boxes that have been shipped.

Guys, I took your advice and strapped them to a pallet. I feel much more confident of their safety now. Thanks