Connect a sub to stereo system

Do you have a second pair of outputs on your preamp?

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.

Thank you everyone for inputs. I will try it today. 

The best way to connect a sub, ime, is coming off the speaker outputs of the amp into the speaker inputs of the sub. It will be more coherent. Enjoy ! MrD.

@ MrD ! How can I do that?

I don't believe your Klipsch offers the speaker cable connection option...

@jl35 no, it doesn’t. 

Sorry. I be mistaken. Enjoy ! MrD. 

Solution = miniDSP
+Solution = miniDSP HD

I connected the Lab outputs to power amp and the Norm to my sub, everything works very good so far, I have more Bass, 

Glad that worked out for you OP.
Now just enjoy the music!

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. 

DEQX

The first and biggest mistake made with subs, is asking/expecting them to do the work of your primary speakers in the area of mid base.
Most good floor standing speakers, including your R28Fs are quite capable above 40hz - don't expect your subs to do their job.
As frequency levels rise above 100 Hz, they start becoming directional - creating a disconnect between your primary speakers and your sub/subs - if they are not time aligned and in proper relation to your mains. This is why some, falsely, believe that a dynamic sub will not integrate well with a ribbon or stat.
Use the frequency setting on your sub set near the lower range of your speakers - in your case, probably 40hz, but not above the 80hz setting. Set the phase switch, on your sub, at which ever position presents the loudest base (running the sub out of phase with your primaries will cause a canceling effect on the volume and impact of your sub/subs). Spend some time experimenting with the placement and volume levels of sub/subs and room treatments, if necessary.
With a bit of time and work, I think you will be surprised at the difference that a well integrated extra octave of deep base can make in the presentation of your music.
Best of luck...Jim

Thanks @