I agree with Sean, generally the corner is the worst placement for a sub as it fully excites three room modes (often the most lumpy response you can get)...it is the best placement ,however, if you only want to maximize your LF response.
The placement Sean describes...just in front of the left or right speaker and next to the side wall is often the best spot in most rooms
The placement Sean describes...just in front of the left or right speaker and next to the side wall is often the best spot in most rooms