I'll probably get some flack for this but when you say you come down hard when the dog is being aggressive, how hard do you mean?
If he were my dog, i would have him pinned to the floor by the skin of his neck, on top of him screaming in his face, so angry my eyes would be bulging, and spitting hell fire untill he screamed in submission I've found REAL anger like this, coupled with the fact the dog is restrained and cannot get away till you let it much more effective than a good hiding.
Don't get me wrong, i don't do this to all my dogs, i train them from an early age i am boss, i do the