I just happened to be reviewing my Hsu Research subwoofer manual and it has a nice synopsis on rules of thumb for placing a subwoofer:
Rules of thumb for placement
• Avoid the center of the room: In general, avoid placing the subwoofer half way between the front
and back walls. This is where you get a strong null from your room’s standing waves. You should
also avoid sitting in that area. No matter how powerful the subwoofer is, there will not be much
bass around the center of the room.
• Use corner placement: Subwoofers usually sound best tucked in a corner. A good corner is far from
wall divisions and has at least 6 feet of wall to either side. If you have more than one good corner, use
the one farthest away from large room openings or the one closest to the listener. Keep the subwoofer
within 1 foot of the wall.
• Use nearfield placement: A good subwoofer usually sounds best close to the listening position. As an
added bonus, the subwoofer’s volume level will be lower so neighbors are less disturbed. We strongly
recommend this method if your couch is up against the back wall and your room is over 18 feet deep.
• Use “subwoofer crawling”: This excellent technique is not as hard as it sounds. The room’s
acoustical reflections are used to your advantage. Place the subwoofer in the listening position, in
a seat, towards ear level. Connect the subwoofer to the system and play some music with steady and
constant bass. Walk around the room, listening for the nicest and most even bass. When the bass
sounds good, crouch down and listen where the subwoofer would normally be. You may use a SPL
meter to measure the evenness of the bass. Mark the best sounding place. The subwoofer should
be placed there.