Our Labs do a good impression of hell hounds when required!
Very steady good pets, fit enough to handle husky size walks. Would slobber someone to death in an instant.
I did have a horrible situation years ago - a man tried to force his way into the house, I'd put the dogs in the kitchen before I opened the door(routine then because that house opened onto a road).
They went ballistic, not just making a noise they really meant business. They could hear me shouting and us wrestling the front door and I remember being stood there not sure whether to worry more about the bloke trying to force my door in, or whether my dogs would break through the kitchen door and maul him. I won in the end and got the door shut, thankfully.
Before then I would have said they would be too laid back to see off an intruder (apart from the noise), but I think they may well have attacked him had they been loose.
If I wasn't home they would probably have shown them where the family silver was in return for a stroke.
Equally I could argue if the dogs had been with me when I opened the door he wouldn't have tried that.
Those dogs are gone now, I had them their whole lives and that was the only time they were ever aggressive. They were steady, friendly and sociable dogs, never a hint of any more of that aggressivness shown again.
My current set would probably boot me out the door for a bit of peace and quiet though.