1) understand your room mode
2) phase
3) placement
4) delay
When subwoofers aren't correctly handled, it will overkill your main speakers. Out of phase can happen between the subwoofer and main speaker. It can sound very loud but totally out of control and has no bass definition causing a chaotic overall sound stage.
The rules of thumb to place a sub by most is said to be corners. Although corners can give you low frequency boundary gain, it is not neccessary the best location to get a flat bass response. Most of my home cinema project do not place the sub in corners. I usually go for dual sub to counter peak and dip caused by room mode and this usually solves the problem easily.
How much is enough? How loud should a subwoofer goes? You take in all the factors and come out with the correct formula and you get the best sounding subwoofer anyone could have hope for.
A quad sub will be a challenge but once you get it to work, you will know what have you been missing all this while.
