I have an apline cda 9857 hu and i want to hook up two subwoofers running parallel. what do i need?

i just bought a car and it had the cda 9857 head unit in and the person took out the two subwoofers installed in the rear deck. i want to spend the least amount of money for decent sound. is there a way i can end up not getting an amp? or do i need one?