Need help determining best shipping practices

Your speakers are not particularly large, or heavy. Elizabeth and Photon46 have you covered nicely, and particularly the recommendation for a double box with 2 inches additional space between the two boxes. That is great advice. Many of the better manufacturers use similar packing.

Below are a couple of additional thoughts from someone who has shipped a lot of gear.
Do NOT use USPS...do not even be tempted by the lower price,
insure for full value and, as a hint,
Vladimir Lamm recommends only shipping his amplifiers via an air carrier and never by ground Lamm shipping recommendtions

If of substantial value, best to use a 'white glove' carrier. They will do their own packing and transportation. Little risk, but expensive.

I have the original factory boxes and foam inserts. I'll price up the idea of extra box+foam or even the 1/8" plywood. I guess you're rolling the dice with any carrier you select. In my research I've read horror stories on this site about all of them.

Agree with Elizabeth's suggestion about double boxing. I'm really paranoid about shipping damage and several times have added a thin plywood liner to the outer box to help prevent the kind of damage that she and Photon46 describe.

If you have the space and tools, go to your local home improvement store and get a sheet of 1/8" or similar very thin plywood. Cut pieces to match the inside dimensions of your outer box and glue them to the cardboard with a hot glue gun or your adhesive of choice. Then add the foam liner.

The thin plywood is cheap and adds very little to the total package weight but adds substantially to the impact resistance. I shipped a very nice Simaudio amp to Bangkok packed that way and the new owner seemed as thrilled with the packaging as he did with the amp.
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+1 to what Elizabeth said. Expect and prepare for ANY carrier to drop your package off a conveyor belt onto a concrete warehouse floor, knowing it will land on a corner. If remotely possible from a financial perspective, two day FedEx Air is my preferred delivery method. Less opportunity for SNAFUs to occur. However, if this thread grows to any length, every carrier will end up getting panned and praised, everyone of them screws up a certain percentage of the time. I have noticed a huge rise in problems with USPS shipping in Florida, not sure if that's the case elsewhere.