The best thing you can do is to experiment. There are many schools of thought on sub placement. (as you can see by reading here) It just takes a bit of effort to try different set ups. You don’t have to spend money to try it. Find something to place them on temporarily and see. I made some cribbing blocks about 14" square out of stacked 2x4 lumber. Its easy to add layers to adjust the height. (Milk crates work good) The added mass helps also. Dennis Foley of "Acoustic Fields" (they design room treatments for recording studios and listening rooms) recommends about 18" off the floor for subs to help smooth out the response.
I had the same experience as steakster and my subs are located just in-board and slightly forward of the main speakers which are 55" from the front wall (to the front of the speakers). This position not only makes the subs integrate seamlessly with the speakers in time but gives much tighter cleaner more tuneful articulate low end. I have tried many different positions and this is the best setup I have found for "my" room and system. Some will surly disagree. That is just the nature of this hobby.
I have found that putting subs well behind the speakers such as in the corners makes them sound somewhat disconnected from the speakers. Low frequency’s are much slower than higher frequency’s and take longer to reach your ears. Putting subs well behind the mains makes this worse. I’m talking about time alignment.
Speakers are Vandersteen 3a signature’s and a stereo pair of Vandy 2wq subs. (Vandersteen’s are time and phase aligned speakers.)
