Anyone do any upgrades yet or plan to? I didn't get the premium audio bc I didn't want to special order or hope for the right car to come in with it. Would like to upgrade the speakers but am not sure the route to take. Just replace factory speakers? Find a way to add a subwoofer? I wish someone would design a subwoofer box to fit the back corners. I'd love the addition but don't want to give up much space.