I would take it by your post that you do not have a sub at the moment, but are lookingt to get one. If this is indeed the case, then before you purchase your sub check out the connections on the back.
There are three ways (and thoughts) on how to hook up a sub:
1 - Most subs will have a line in and a line out using the same type of cables that you presently use to connect your pre-amp to you amp. Simply get another set of cables, then go from your pre-amp output to your 'line in' (or 'sub in', whatever it is labeled), the go from the 'line out' on the sub to your main amplifier. Be careful to maintain proper polarity (left to left, right to right). Most subwoofer do not have a true crossover built in, so your speakers will still be running full range; the crossover setting just determines the upper frequency of the sub.
Some subs do have built in cross-overs, like my REL Stratta II, but not all do. Look at the documentation (or manual) before you buy if this is going to be important to you.
2 - If the sub has speaker level (ususally called 'hi level') inputs, then simply run an additional pair of speaker wire from your amp to your sub, leaving the speakers still connected to your main speakers as they already are. Again, you main speakers will still be running full range.
3 - Get an external electronic cross-over. You would then go from you pre-amp to the cross-over, the crossover would then 'split' the signal, with the low frequency going to the sub and the higher frequency (as determined by the cross-over) going to your current amplifier. The benefit of this method is that now your main speakers (or amlifier) do not have to reproduce deep bass, that is what the sub is for, and you should see [potentially] an increase in dynamics, power and resolution.
Everybody has their favorite method, with method 3 being the most popular. My 2-channel set-up uses method 2 with a very low cross-over point on the sub (30 Hz). That was the best blend that I found after several weeks of playing around.
Whichever way you go, a good sub on a 2-channel rig can make a big difference in overall sound. When properly set-up, you will wonder how you ever lived without one. Just remember, 2 things, subs are not just for HT and the best sounding sub is the one you don't hear, that is until it is turned off.