We've owned big dog breeds for decades. They always look to me for keys on how to react.
This talk of being seen as alpha/boss, it doesn't mean you need to always be muscling and challenging them most times, it's as simple as lifestyle.
From puppy on up, any time one of our dogs want something--whether it's to go outside or have a treat--I always make them perform a task on my command before they get what they want.
Scratches at the door to go outside? Sit after I tell you to sit.
Want a piece of this snack I have? Sit and shake on command.
It's a wonderful way to establish respect and pecking order in a healthy way, while also teaching and encouraging self control.