Question I Have Never Seen Before?

Consider having a piano moving company ship them. When I've moved my speakers (2 x 240 lbs & 2 x 350 lbs), I've included them on the truck with the piano. No box needed: They just stand them up in the truck, wrap them in blankets, and bind them securely to the inside walls of the truck.

Buy some really sturdy Uhaul boxes like "dish pack". I cut them completely in half lengthwise. Wrap the speakers in pounds of bubble wrap and then assemble the boxes with a couple of pounds of tape. Cutting them in half allows you to squeeze them to fit.

Call your local crating company they usually carry double walled boxes. But beware, I would not ship something like that unless they are small. Thats just crazy and risky.

I would buy a roll of shrink wrap and wrap the living crap out of them and put the shipping label on it before tryin to do boxes. I have done that before.

I shipped heavy Reference 3As (in boxes) by Greyhound once. Worked well and was cheaper than FED/EX or UPS.