Don't add a second sub unless you have a VERY LARGE Room. It will make the Low End worse. Some tend to think otherwise and they regret it. I used to be a Forum regular on Sound and Visions Forum, but grew tired of the cookie cutter replies, brand bias instead of research, etc.
I don't think this works as a blanket statement, matching the sub to room is very important. Placing the sub in the room is equally important, two poorly placed subs can make things worse, but two well placed subs in the room can help sooth out the bass response in the space and reduce the number or peaks and valley in the acoustics of the room. Multiple woofers isn't about making things louder it's more about compensating for the room they're in.
The quick and easy way to determine best subwoofer placement is to put the sub in the primary listening position and then walk around the room (or better make a fool of yourself in front of your family and loved ones and crawl). Play something with a repetitive or continuous bass line and find the position where it sounds best and put the subwoofer there.
As far as subwoofer (home) recommendations I'm currently a fan of Velodyne (
http://www.velodyne.com/main.aspx. I recently had a meeting with their regional sales rep who took me through the line and it's very thorough with lots of subs for different conditions and at many price points, subwoofers is all they make and they do a pretty bang up job of it.
