PREHEAT oven to 350°F. Combine flour, baking powder and baking soda; set aside. Beat cream cheese, butter and 1 cup of the granulated sugar in large bowl with electric mixer on medium speed until well blended. Add egg yolks, one at a time, beating well after each addition. Add flour mixture alternately with the sour cream, beating well after each addition. Add poppy seed and orange zest; mix well.
BEAT egg whites in small bowl with electric mixer on high speed until stiff peaks form. Gently stir into cream cheese mixture. Pour into greased 12-cup fluted tube pan or 10-inch tube pan.
BAKE 50 min. or until wooden pick inserted near center of cake comes out clean. Combine remaining 1/2 cup granulated sugar, the liqueur and orange juice in small saucepan; cook on low heat until sugar is dissolved, stirring constantly. Prick hot cake several times with fork. Pour syrup over cake; cool 10 min. Loosen cake from side of pan with spatula or knife. Invert cake onto wire rack; gently remove pan. Cool cake completely. Sprinkle with powdered sugar. Transfer to serving plate. Garnish with quartered orange slices, if desired.