Cream butter and 1 1/2 cups sugar until light and fluffy; beat in egg yolks one at a time. Fold in beaten egg whites. Sift together flour, baking powder, baking soda and salt, and add to batter a little at a time, alternating with sour cream. Stir until smooth. Mix in grated orange peel and walnuts and pour into buttered 9" tube pan. Bake 50 minutes at 350 degrees. Mix remaining sugar with orange juice and Amaretto and spoon over cake as soon as it comes out of oven. Cool before removing from pan.