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.
There are 2 pair output from my preamp: 1 is “Norm” and another is “Lab”. I am using the one labeled “Norm” to my power amp. So can I use the other output labeled “Lab” to my Sub?
Actually the manual states that the outputs labeled "lab" are for the "more critical listener" and allows the full frequency range.
So I would say you should use the outputs labeled lab to connect to your power amp and the ones labeled norm to connect to your sub
Although once you hook up your Klipsch speakers to the power map connected via the lab outputs you may find you have all the bass you actually need.
I think the lab setting just by passes the tone controls and the norm has tone controls in the signal path so I agree with uberwaltz
The difference between the two outputs is that the "lab" is direct coupled and the "norm" is capacitive coupled. I agree, use "lab" to the power amp.