All subs should be off the floor by approximately 27-31 inches. This idea is from ASC (the tube trap people). It works by minimizing standing waves as low frequencies and providing better coupling of the woofer drivers with the room. ASC makes a special tube trap to elevate your subs off the floor to take advantage of this idea.
I should note that the height off the floor and distance into the room can vary depending on ceiling height.
I should note that the height off the floor and distance into the room can vary depending on ceiling height.