Need help determining best shipping practices

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.

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

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