How do pack a pair of large speakers

Carefully.

strong outer protection (box) speakers are secured inside the box with thick surrounding ( of bubble wrap..) Themocol/sponge spacers (3-6" thick) which keeps object in place no matter what. securing Corners are very important. Plastic cover to take shower. Imagine what abuse box is going to get during transport. Handlers not aware of how fragile, sensitive or expensive is product inside. Handlers may throw from 15feet high. Box may sit exposed to all elements. Forklift may brush the carton.

I recently moved and had the moving company do "custom crates" for heavy, expensive monitors. Turns out they charged $300 bucks for it and all it was was a guy taping some cardboard he cut on the fly to the speakers. Fortunately they weren't damaged and I got my money back but definitely go with original packaging or pay service that does this and only this.

The usual 2 ways....

Badly or correctly.

Will likely cost about the same if the shipper even considers the first...perhaps more on the advised insurance...

What model of speakers are they?  This would determine how large and heavy they are and if they need to be crated rather than having cardboard boxes.