I recently had a problem with an amp head that got damaged because GC didn't package it very well. GC gave me the option of returning the amp for a full refund, but I asked them about having the amp repaired locally, keeping it, and getting reimbursed for the trouble.
I got a quote for the cost of parts delivered from the manufacturer, and I had a factory-authorized warranty center write up a repair estimate. I took the parts invoice and the labor estimate into GC and they cut me a check to cover the damage. It took all of about 5 minutes once we had someone on the case who knew how to beat the computer.
I wasn't smart enough to ask for an item at cost to make up for my trouble. Drat.
If you want to keep the cab, you should be able to do the same thing I did, just call the GC store and talk to the manager. You can order replacement speakers from the Mesa web site and have them shipped to you, take the bill to GC and get reimbursed by GC. It's a pain, but if you like the cab it's worth the effort.
If push came to shove and you couldn't work out a decent repair option then I'd return the cab at my local GC and ask for a full refund including tax and shipping. Since the problem with the cab is not your fault they should be willing to accommodate you. If they balk at the idea of refunding your shipping charges, just ask for whatever return you can get, get a receipt for the return at your local store, and call your credit card company. VISA will refund shipping charges if the item isn't what you ordered, and in those cases you're only responsible for the return freight to the merchant. Returning to a local GC store makes your return shipping charge almost nothing, so you get a 100% refund from VISA.
My first choice would be to work with GC to get the cab repaired. They really did a good job helping me in my case, because they were motivated to avoid taking a total loss on a wrecked item, they wanted to preserve a high dollar sale and they wanted a happy customer. Turth be told, I was amazed by how easy it was to fix the problem.