Thanks for yours feedbacks.
It is definitively a design flaw.
I think that the earcups of over-ear headphone should rotate a little bit forward to fit the head. I know the head are different for different people, some people may be totally fine with the angle of hp50. But why NAD do not respect the general anatomy.

I don't think the small variation of angle could play a big role here. I twisted the angle with my hand but nothing changed. When i wore it left on right, the soundstage do heard weird because sound felt like coming from behind. I then reversed the stereo channels of the source, everything came back to normal. The room feel technology give hp50 a beautiful 3d-like soundstage. They have done an effective psychoacoustic research.
I just ordered some cables to make a channel swapping cable. I hope it will work out well.