Question I Have Never Seen Before?

U-haul has heavier grade cartons for TVs and other electronics. Not much more than the cheap crap they sell at Kinkos. I would use rigid or semi rigid foam to line the boxes, top, bottom and sides, then a double wrap of bubble wrap around the speakers, with extra on top and bottom. They should be packed to survive a 6' drop onto concrete! really!!!

UPS store can pack and ship, it will cost a bit more but it wont be denied because of poor packing if its damaged in shipping.

Start by going to your local big box retail store and asking them for some large sheets of cardboard from a flat panel TV or appliance and also see if they have some heavy duty cardboard corner pieces that are used to ship fridges safely. Wrap the entire speakers with several layers of cheap cellophane wrap (Saran wrap) to protect the surfaces of the speakers. Attach the cardboard corners to all edges of the speakers and make a box out of the sheets of cardboard to use as your "inner" shipping box, then surround that box with 2" of white styrafoam (available from your local home improvement store for $10 per 4X8 sheet) and then make your final outside box around the styrafoam. It sounds like a lot of work, but if they're not packed in this manner, FedEx or UPS will deny any damage claim due to "insufficient packing".
Dave

Definitely Kinkos/Fedex packaging service. They took care of my sound anchors speaker stands that weighed 90 pounds each with no problem at all.

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.