Most economical way to ship large speakers?

Klipsch uses Old Dominion. 

You could also look at Greyhound freight or an independent shipper from some place like UShip. 

Reminds me of the quote by Trek Bicycle's Keith Bontrager - "strong, light, cheap, pick two" 

"Reliable and economical" are somewhat at odds with each other.  Any of the shippers are ok - until something goes wrong, then economical goes out the window.  If they are truly large (implied heavy) speakers, and especially if they are expensive, I would only ship them by strapping them to a pallet, or pallets, and using a reliable freight carrier.  FedEx has a freight division that I have used for large amplifiers (Lamm) that were strapped to pallets.  If for some reason you don't want to do that or it is too expensive, then at least double box with at least a couple of inches of polystyrene foam between the manufacturer's box and the outer box, if possible.