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.
justinpham2012
I connected the Lab outputs to power amp and the Norm to my sub, everything works very good so far, I have more Bass, 
You could try an external crossover like the JL Audio Cr1. The preamp output would go to the crossover, the crossover would then connect to your amp and sub.