I have found that if one sub is to be used, it should be downward firing. This is another level of defense, in detecting position of the lone sub. I have also found that placing a sub exactly in between the main speakers is not a good place, as it creates a pressure ridge that disturbs the imaging, at least with my Martin Logans.
That same pressure ridge can be had with older style, vintage speakers that have mid-range or tweeters offset from the woofer. The cure is to simply remember to place the mid-range/ and or tweeters to the inside.
With the down firing sub placed on the outside, it is now possible to work it around the nearest corner, as it too is an amplifier, as each surface (floor, rear wall, side wall) can add 6db of gain (naturally) to the subs output and also help diffuse the apparent location. Now the sub is probably already on the floor, so there is one unit of gain there, moving it closer to the rear wall, will increase output again, and then sliding it towards the corner will result in another increase in bass output.
I have also concluded to myself anyway, that overlapping cross-overs really muddies up the low end and makes the system bass heavy (unless you love bass). I tell friends to remember that subs are really just a "one note wonder", as they barely cover a couple of octaves (40-80-160). Our ears sense of pitch is very insensitive to such low frequencies, but this is the region where the sense of "power" begins.
My 2 cents
W
