adding a sub

i recently purchased paradigm studio 40 v4's. I feel i'll need a sub, possibly 2 small ones. My question is this: my amp has only binding post for right and left channels. My pre only has 1 pre-out. Where do I hook up a sub? If I hook it to 1 channel on either side how does that affect the speakers?amp? I may get 2 small subs just because all of my stuuf has to be symmetrical or I freak out a bit.