I used to work for Mail Boxes Etc, before they changed into the UPS store.
They both have their advantages, it depends on what you're shipping.
FedEx is better for light fast shipments. Like their next day letter packs, or small boxes. They come it at a decent price compared to the others.
UPS is very good for large bulky items, in fact they have special handling requirements for heavy and large boxes.
I'd recommend going with UPS for the speakers, and putting a this side up and fragile sticker on the side. That'll make them stack it towards the top of the truck load. Also, for insurance purposes, just in case they do break, make sure to have at least 2 inches of packaging around the speakers inside. And don't use newspaper!! It automatically voids any insurance you put on the box for both carriers.
PS, you get a free $100 of insurance per box from both UPS and FedEx.