PSB Speakers PSB SubSeries 500 Guide D'utilisation page 9

As you go outward from the corner along one wall or another, the general
consensus (with which we tend to agree)
is that while bass output diminishes somewhat, it also becomes more
uniform throughout the room, with fewer of the "standing waves" that
produce peaks and nulls at various points.
Chances are things won't be so simple, so the best method for
positioning a subwoofer, although a rather undignified-looking one, is to
put the subwoofer in your listening chair, then play music with lots of
bass through the system something with steady low frequencies (such as
organ music) or continuous test tones, not movie material. Move around
the room and note where the bass sounds best; if you place the
subwoofer there and yourself in your chair, you should get the same
bass performance. Bear in mind that the test only works if you have your
ears as high off the floor as the subwoofer will be, so don't be afraid to
crawl around. A recommended starting point for the placement of this
subwoofer would be in either of the front corners of the room (on either
side of the main speakers).
Multiple Subwoofers—Why Two Subs Are Better Than One
Since the objective of most people who buy subs is to make sure of
plentiful low frequencies, the only situation most of us will run into that
makes subwoofer placement really difficult is the factor we all fear—the
"bad" room that just won't let you get satisfying amounts or quality of
bass. There are rooms with troublesome dimensions, especially as you
approach a perfect cube (with a closed door). There is unlikely to be any
combination of speaker and listener position that will be free of obvious
acoustic anomalies.
In such a case, the best way to iron out those anomalies is with two
subwoofers, placed carefully to work with each other. This can also be
true when the problem is too much, or too uneven, bass. The overall
system needs all the help it can get, and that often means the use of two
subwoofers, each one of which corrects for the acoustic problems
excited by the other. For excellent results from this solution, the two subs
don't have to be identical. It may be fine, in fact, to use two lesser subs to
equal the performance of one with stronger specs.
The same "crawl around the room" method as previously described
should be used for determining the location of the second subwoofer,
except in this instance one is listening for the minimum amount of bass
output. This is a recommended starting point for determining the best
placement for your subwoofer(s).
Setting the Subwoofer Controls
Once a reasonably smooth response has been achieved by careful
positioning of the subwoofers, the overall performance can be fine-tuned
by means of the controls found on the speaker. An important one is the
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