Cheapest way to ship large speakers?

You might try freightquote.com.

If possible, you want the same company handling the shipment all the way to the buyer's door. If there are two companies involved, each will blame the other if there's any damage.

Can you ship these as freight to a commercial address? That will be cheaper and probably safer too.

Freight. Pack the speakers individually. Get a correctly sized pallet and strap them down laying side by side with extra layers of protection to reduce the chance of a fork lift damaging them. I had some speakers shipped to me that were about as you describe from Chicago to Calif. for 3 or 4 hundred.

I would not ship them unless they were packed in their original box with the original packing material. Any other way you'll be asking for trouble.

I'd also say freightquote.com. I've used them without issues and you can choose the least expensive of the quotes.