Dobies are a dominant breed by nature. Another thing you might add to what you're already doing is to go over and lavishly praise the pet who was growled at and give that pet your attention in front of your doberman mix while ignoring the dobie's attempts to "make up." I read that after punishing your pet you shouldn't make up to them right away, but ignore them for a period of time, because a punishment won't mean very much to them if they know they can be back in your good graces immediately afterward. That came from "How to Be Your Dog's Best Friend" by the Monks of New Skeete. (They are long-time dog trainers of German Shepards, another dominant breed.) I think that making your dobie stay off the couch is very good advice too. Other than that, you might call a dog behaviorist, or start reading all you can find on dog pack hierarchy and dominance/submissiveness. There are lots of books by dog trainers that can be found in the library. Good luck.