Originally Posted by rcohen
Here are a few suggestions to try:
1) One in each corner, time aligned (most power, canceling some room modes)
2) At 1/4 & 3/4 positions on the front and back wall, time aligned (cancels additional room modes)
3) At 1/4 & 3/4 positions on the front and back wall, but invert polarity for the back pair, and add delay to the back pair of a bit less than 1 ms/ft for the length of the room (Double Bass Array - cancels most room modes)
Midwall placement has been documented as one of the very best positions of subs. Best if you can have all four subs on their own wall, but if you want to stack them, front/back or left/right wall is also good. Always use opposing walls, if just going with two walls.
If you do sidewalls midplacement, do not set XO higher than 80Hz due to localization, but honestly I don't think you will localize a F15 even at XO=100Hz, but this is something you can try out.
My recommendation is not to invert polarity for a backwall sub, you might get it look better at the XO freq, but it might get worse at other freq., it has not worked out for me to do it like that. If you set a distance to the sub in your AVR, set the distance to the sub farthest away, then the phase on that sub could be 0 on the sub phase dial. Then you delay the sub that is closer to you at MLP on the sub phase dial. If subs are at the same distance you can have both subs at 0 and just play with the AVR phase/distance adjustment.
Not sure if you already have, but just in case, with more than one sub you really need a measurement mic and a SW (like REW) to make the best call on placment and phase adjustments, no matter how golden year you are, you are likely to get a much better integration by doing simple freq. sweeps and see what happens when you play around with placement and settings.