Butter and flour a 10-inch tube pan. Heat oven to 350°.
In a large bowl, measure the sifted flour, baking powder, salt, soda, and orange zest; set aside. In a large mixing bowl, beat butter with 1 1/2 cups sugar until creamy; beat in cream, vanilla, and eggs until light. Slowly beat in the flour mixture until well blended. Stir in the poppy seeds.
Spread the batter in prepared pan. Bake for 50 to 60 minutes, or until a toothpick inserted in the cake comes out clean.
Meanwhile, about 5 minutes before the cake is done baking, combine Grand Marnier, orange juice, and 2/3 cup sugar in a saucepan. Bring to a boil, stirring constantly. Lower heat and simmer for 1 minute.
Take the cake from the oven and place on a rack; poke all over the top (dozens of times) with a long skewer or cake tester. Spoon the hot syrup mixture over the cake. Let stand for 10 minutes then remove pan sides. The syrup should have soaked in completely. Let the cake cool before carefully loosening and removing the tube section.