Combine first 6 ingredients in a heavy saucepan; bring to a boil, and cook, stirring constantly, until sugar dissolves. Add 6 lemon slices; reduce heat, and simmer, uncovered, 5 minutes, stirring occasionally. Remove and discard cloves, cinnamon sticks, nutmeg, and lemon. Remove from heat; stir in rum and amaretto. Garnish each serving with a lemon slice, if desired. Serve immediately.