Blend zwieback crumbs, quarter cup sugar, and the 6 tablespoons melted butter. Press firmly over bottom and up sides of a lightly buttered 23 cm spring-form pan. Chill. Beat the cream cheese until smooth. Add the 0.75 cup sugar and the 0.75 cup brown sugar, beating until well mixed. Beat in the eggs one at a time, until mixture is light and fluffy. Beat in the pumpkin pie spice and the heavy cream at low speed. Mix in the pumpkin. Pour into prepared pan. Bake in a slow oven (325) for one hour and 35 minutes. While pie is baking, mix the topping ingredients (the last 3 ingredients), first the butter and brown sugar until crumbly, then blending in the nuts. After the one hour and 35 minutes, remove the pie from the oven. Spread the topping over it, and return it to the oven for 10 minutes. Remove from oven and cool on a wire rack. Refrigerate for several hours, or overnight. This cheesecake is rather large, and incredibly rich. Everyone always wants more than they can fit in their stomach! And the recipe! :-) Serving suggestion: Some like this garnished with whipped cream and more walnuts, or with whipped cream and pecans.