Well, the corners are going to get seriously mangled if you don't pad them heavily. Use hard foam on the corners, and stick a few pieces on each side of the speaker. Pack the rest with bubble wrap (make 2 or 3 foot pieces of it and fold them in half). I would then double box that, just to be safe.
If you reeeeeeeeally wanted to be safe, go to Home Depot and ask them if you can have a pallet from the back (offer them $10-$15 bucks maybe if they won't give one to you). Strap the boxed speakers and grills to the pallet. This way they will be forced to fork lift the entire thing AND it won't get tossed around during shipping. Make sure to put "Do not stack" on top of the boxes as well.
Also, Fedex ground really sucks. Never use them, your package will be completely destroyed unless you put it on a pallet, then it has a decent chance of survival. (for the love of god, insure them as well!) I worked in the shipping department of a high end audio company for a while, so I've had a lot of experience with this.... let me know if you need any help.
Edit: I read you were ebaying these things. Well, I understand the desire to keep costs down, so our ebay policy was as follows for large items: We provided you a shipping a quote that we believed was cheap AND a safe way to transport the item. If the buyer wanted less, that was his perogative. We would warn him/her that we didn't think this was a safe way of transporting the item. We ALWAYS forced people to insure their packages, nomatter what.