Any solid wood will sound better than any plywood, and that's not just another internet theory. A local guy here is known for his hand built '59 Bassman's. The first one he built, his personal one, was what the original specs called for, which is a pine box. Since building the first, he's built a number of them where the customer requested birch boxes because they wanted a stronger box. Yes, birch is definitely stronger, but they don't sound anywhere as good as the original pine box model. I've heard them both many times, and I wouldn't do it any other way, pine box is the only way to go.
However, if you're playing metal, I don't think it's going to matter..