Connect a sub to stereo system

Hi! Everyone! I am seeking advice/solution to connect a subwoofer to my stereo system. I have a preamp Adcome GTP-500II connect to CDA-250C power amp then to a pair of Klipsch R28F. I also have a Klipsch R-12SW sub, how can I connect this sub to my stereo system? 
Thank you everyone in advance.