Then you can either chain them in between the amplifier and the speakers (using speaker cables)...
OR...
Plug them into the back of your preamp via the RCA/XLR connections.
Using the RCA/XLR connections will maintain full range sound to your speakers.
If you chain them in via speaker cables, most subs (you haven't specified what you have) will play the sub-bass and then pass the upper frequencies to your speakers.