
Personally, id advise you to find different trainers.
There are many reasons why he may be barkign at them, but him looking at you as a pack member rather than a leader aint gonna be one of them, because dogs do NOT think humans are other dogs, thus they DONT think you are a member of their pack in such a way as your trainer would have youbelieve.
This is where all this 'dominance' cr@p comes from and it really DOESNT help you train your dog.
Dobes ARE big strong pushy dogs, as with all dogs, they need to know where their boundaries are, whats acceptable (ie will be rewarded) what isnt acceptable (ie what wont be rewarded).
If your trainers do not have his respect, and they need to EARN that, not scare him into it, then its likely he WILL jsut stand and bark at them rather than obey a command.
But then its as likely that he is confused, doesnt understand the question, or is unnerved as anything else.
If he behaves in a manner which is acceptable to you in your home and in your company, then that is fine (as long as it doesnt upset other people obviously).
Em