Plug a speaker cable in the cab, touch a 9v. + side to the tip of the cable, the - side to the other part and see if the speakers move outward, all of them. This will at least tell you if they are hooked up right. It sucks to do but gently move the cone and see if it flows gently, not scratchy. Be gentle though, most will say not to do this but it lets me know if the damn thing is blown.
Edit, did I mention be gentle enough? good luck man!