In desperate need of help for training my Lab/Pitbull mix.?

My fiance and I have had this dog since November of 2010. We rescued him from the pound. He was one of a litter of eight male puppies and we were given no information on the parents. He looks like a lab mostly but some people say he has some characteristics of a pitbull as well. My fiance and I DO NOT want to get rid of him, but we are at a loss of what else to do. This is the worst dog we have EVER had. And we have tried everything from cage training to the clicker to a shock collar to a sonic sound remote. Nothing works. If you reprimand him, he actually barks back at you. He doesn't listen AT ALL to anyone telling him to stay off the couch or not to eat food off the table or anything. He bites hard, and when I lay on the bed, he actually bites the back of my neck and my fiances neck. Spanking doesn't work, he still barks and bites back. My fiance and I definitely want children sometime but neither of us trust this dog enough to even let him near OTHER children. If we do get rid of him we can't even consider giving him to a family because he bites. We both love this dog to death and want it to work but he is destructive and has destroyed a leather couch we haven't even had for a month that we still need to make payments on. We live in an apartment, but he goes for daily walks and occasionally goes to the dog park or a friends back yard when we go over for barbeques. We put him through a training course at PetSmart and he failed MISERABLY. He was the barking trouble maker in the class. We got him the shock collar and he learned that he only gets shocked when he barks loudly so he started just moaning quietly enough so that it was still annoying but not shocking him. I am with him all morning and my fiance is with him all night. What can we do?!?!?!