I have never had a LGD, and I understand that their issues are as different as their temperment from other dogs. That being said, I have had dobermans all of my adult life. And let me tell you, the right (or wrong) doberman will have you down and disabled in the time it takes you to think about getting away. My first male had some issues with aggression, I had three episodes in which I stopped him in full attack mode when he perseved a threat to his "pack". He was never trained as a "guard dog", he just followed his instincts.
I kept two of his pups, my sister-in-law had two, and I have knowledge of 8 others in the area, knew them on sight and spoke often with the owners. Of all of his git, NONE of them had his aggressive tendencies, all twelve of them were mellow family dogs, no more unpredictable than your average lab. Half of the battle is environment and training, genetics is only a part of what makes our dogs the way they are. The tendencies may be bred into them, but how you manage those tendencies can win the war for you. Samson has a better than average chance of being just FINE. You have already worked through so many issues with him, and have such a stron bond that I am confident that he could not be in better hands.
Stella