What breeds are predominate in my mutt?

I’ve got a new puppy and I’m wondering what breeds he might consist of. I got him at about 3 months and he weighed 9 pounds. Now he’s 6 months and weighs 35 pounds. He’s very smart and trains easily–housebroken after 2 weeks and he can now sit, stay, fetch, find toys, etc. He’s extremely strong for his size. He’s energetic, but not hyper. He’s very quiet except if someone comes to the house. The minute he hears the door he barks and growls and it’s not his usual “puppy bark”. He can make himself sound like a huge dog that will tear you apart. When we’re out in public he never barks at anyone. When I got him at the pound they thought he might be a black lab mix. However, as he gets older he doesn’t really have the lab look. His head is much more narrow and his coat has picked up a lot of brown. He’s got short velvety fur but he’s got a patch of rough, wiry hair down the center of his back. I’ll try to figure out how to post a photo…