It would shirk to ask the Prophets directly while knowing they are a Waseela to Allah? Or it would be shirk to ask the Prophets directly thinking they are the ones alone who will fulfill your request? Or both? so the question arises how loud do you have to scream next to the grave of the Prophet in order for him to hear your Dua? what about this verse in the Qur'an? "O you who have believed, do not raise your voices above the voice of the Prophet or be loud to him in speech like the loudness of some of you to others, lest your deeds become worthless while you perceive not." Now there are guards, doors, walls, thousands of people around... It wouldn't be possible to speak in a normal tone of voice and have him hear it, while still being sure you are not going against the clear Nass above.