Subs - Installation

I am in the process of adding 1/2 subs to our 2 channel system.   I am not entirely clear how you install/connect the subs.  I assume they connect to my preamp.  Is the bass still going through my speakers (B&W 804/3) and the sub is adding or is the sub taking all the low frequency?  What is the thought of placing subs next to the speakers.  I am looking at JL Audio Fathom 112 v2.  I see that JL Audio also has an external crossover  I am not sure what this does.  Thanks for the help.