A Subwoofer is a speaker that specializes in a deep bass that creates a more realistic sound. It is also used by some in their cars to create an impressive, head turning display of their music.
I have them and honestly there is not "magical" spot. They work well where ever you place them. They will get the job done. Just don't place them under your chair or sofa LOL
To find out about home theater subwoofers you can go to cnet dot com. Cnet has a wide variety of professional and customer reviews available for a great variety of items.
Sony is one of the brands with the highest reviewed home theater subwoofer. Atavia although a little less famous has great quality subwoofers. Hope you enjoy your home theater.
There are many places that sell home theater subwoofers. You can purchase them new or used at a very good price. Any electronics store should carry subwoofers, and they should be priced competitively.