Turn down a plug of brass or mild steel to the inside diameter of your tube. Cut a rounded taper on one end and thread this end to receive a push/pull rod. Remove the tube from the pelter. Slip the rod into one end until the plug hits the dent. Apply light pressure on the rod, holding the tube in a support that doesn't crush it, like two maple blocks cut for the purpose. As you apply pulling pressure on the plug, lightly tap the dent and the area around it with a brass hammer. The steel of the tube will flow as the dent rises, and you should be able to remove it entirely. Do not use heat, or you'll enlarge the whole tube in the area of the heat.