Dual Subwoofers: close together?

Placing the subs on the front wall inside of your left/right speakers is definitely an option for you.  Sometimes you have physical limitations on how you can place your subs.  But be aware that you can face certain problems.  Having the subs place in the middle of the wall instead of in the corners will mean that the subs themselves will not be as loud.  When placed in the corner, there is a certain about of decibel boost because the sound waves are loaded in corner (essentially, the corner acts sort of like a horn, boosting the bass). 

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Secondly, when you do not place the subs in opposing corners, you are not allowing the subs to combat room nodes.  This is when the bass in a certain size room reflects off the back wall and cancels out new bass frequencies coming from the sub.  The result is a "NULL" or completely loss of that bass frequency in a specific location of the room where the null cancellations are occuring.  This can happen in different places and distances in the room.
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The best way to fight this is using tuned membrane bass traps in certain areas (such as side wall, back wall, corners).  These are tuned to a certain frequency.  The selection is entirely dependent on the size of your room (width/height/depth).  The bass traps are the GIK Scopus traps and are typicall $200-240 for a 2 foot by 2 foot trap.  If you have an odd sized room ("L" shape) or an open room, then it becomes difficult to determine, but those rooms are usually not as affected by bass nulls.

The benefit of multiple subs is the ability to equalize the bass energy in the room thus eliminating or smoothing out nulls and peaks. Experiment with placement is the key as no two rooms are the same.  You can also invest in an inexpensive SPL meter and test record/CD to help measure bass response in different sub placements. 

@brf is right

But if you can afford a HT system and dual subs, splurge a little and get something like OmniMic. :)

There’s a secret though. Put the sub in your listening location, then move the mic around to try to find the best sounding speaker location you will accept. Once you have it, play the first sub, and move the mic around to the second best possible location until you find the worst sounding place. This will put the sub in a complementary position to the first.

Bass traps really help in giving you more placement options, and EQ's are usually still needed. 

Best,

E

I placed my two SVS SB2000 subs just inside my main floor standers and to either side of my equipment rack and this works very well. They are separated by about 6 feet. Lenrd bass traps by Aurelex in the corners of the room stacked three high help tightness and clarity of bass response.