Hi nkist,
Max sounds like a good dog and I see what you mean by him being over-protective.
All my 3 dogs are very protective and barks a lot when someone comes at the door. They are very good watch dogs and I like them being alert like that. However, once we brought our guests in they stop barking and don't jump at the guests and act just fine.
Do you have visitors often? Do you have friends living nearby?
You said Max is fine out somewhere else but at home so I would suggest you have your friend(s) come visit you as often as possible, even every single day for a week or so. Ideally someone who likes dogs and have him/her offer dog treats to Max every time he/she comes. Don't force Max to get close to him/her right away. Wait until HE comes close to your friend. Just repeat many times so that he will learn to get socialized with that person.
It might sound like a long process but he will have to get used to new people one by one to build his trust and comfort level to those visitors so that he will learn that visitors are just fine and he doesn't have to make a big deal about them.
He sounds like getting along well at the vet/kennel so I don't think it will take too long. Also, make sure you praise and give treats to him whenever he does better at the door.
Another thing you can do is to have him sit/stay when you bring in your guest and treat him for being a good boy. Then, have your guest give him a treat as he/she squats down to Max' eye level if needed.
Hope this will help. Good luck.