Before my love for Danes, I used to own Border Collies. I always thought they were some of the smartest dogs on the planet (which they are), but Duke is just as smart if not smarter than any Border Collie I ever owned. He is very smart... thick headed and stubborn at times, but very very smart. I'm sure your Dane will be the same way.
Word to the wise when he gets waist high. Watch your genitals. He tags me about once a week with that giant tale of his.