> next I am going to build set of rear channel for AC-3, as soon as we
 > see a definative answer as to the amout and avg freq of bass
 > information we will be getting in the rear channels. No one knows
 > yet. Many of the high end designers like Duddleston @ Legacy are
 > waiting until the smoke clears and not offering 
 > anything different than their THX rears in di/bipole. And it will
 > probably be a few months before we see anything we can count on.
     Most everything  I've read on subwoofers and placement suggests that
using multiple subwoofers (for below 80 Hz) only leads to increased
room response problems.  Overall, it will make you room response much
more uneven since the interaction of the subs will cause all sorts of
wave addition and canceling.  Two subs are somewhat more predictable,
but adding 2 more for the rear channels is really asking trouble in
the really low bass frequencies, which are usually starved to begin
with.  You might be able to get even bass response at the "sweet
spot" with some real finaggling, but don't expect anything even close
to a flat bass response in other seating positions, especially when
all four (5 or six even?) subs are operating at the same time.
     In fact, one article even suggested that if you already have 2
subwoofers, that the best place to put the second sub is in the same
corner with the first one.  That way their waves coinicide and you get
the extra 3dB of gain without the headaches of standing waves. 
     Stereo bass (below 80Hz) is largely a myth.  Most material tends
to become mono at really low frequencies anyways and I've yet to see
someone determine directionality below 40Hz even with pure sine tones.
80Hz can be localized, but usually only with pure tones.  I've got a
lot of heavy bass material (not rap or techno either) as you know from
reccomendations from Mark and what not and let me tell you, DEEP bass
can image from the other side of the room even because the spatial
clues are higher in frequency.  I only have 1 sub in the front left
corner and have no real bass imaging problems even though I'm crossing
from the sub straight to the 48" ribbons.
     I see no reason why it would be any different with the rear
speakers in an AC-3 or otherwise stereo surround system.  When I say
bass that low in frequency is not directional, I mean not just left to
right, but front to back as well.  As long as you have a sub that is
capable of the levels, speed, and accuracy you desire, you should
really only need (and use) one.  It will give you the most
consistantly flat bass response possible.  Just make sure your rear
speakers can play *flat* and achieve satisfactory loud levels to at
least 80Hz.  This is one of the few instances in audio where the
"saving money" option is actually more desirable.
   Here's a sinewave test you can try with your Legacys some time.
Start fairly high at around 6000Hz or so and play it in stereo.  Then,
while it's playing, move your heard a little from left to right.
Notice the small cancellations going on?  It gets ever so slightly
quieter and louder as you move your head linearly.  Now, keep dropping
the frequency lower and lower.  Notice how the spacing of the various
dips and peaks get longer so that there's a larger distance in-between
each dip and peak?  That's wave canceling and adding going on at all
frequencies.  As you go lower in frequency, the the problem becomes
broader so that one part of the room is essentially one large dip and
another a peak with a flat zone somewhere in between.  At higher
frequencies, it's not much of a problem since they're so closely
spaced together.  If everything is aligned right, there should be more
or less a flat response at the lisenting position over all the
frequencies (don't move your head! :)  But at the lower frequencies,
the effect is more pronounced.  Notice, this is in stereo.
     Now, if you were to remove your Legacys from the equation below
80Hz and use just one sub to cover that area, you'll find there is no
wave cancellation/addition happening since there is only one wave and
no other to interact with it.  Your only problem then is the fact the
length of the wave itself might cause some dips and peaks in the room
length wise.  Putting the sub in the corner helps eliminate these
standing waves since they no longer have parallel surfaces for these
waves to develop between.  So, that combination is what gives you the
flattest possible bass in a given room.  I just ran a sinewave test
prior to this paragraph to verify the former effects.  I *could* tell
directionality between 40Hz and 80Hz with pure sine tones, although
it was not "point" specific (more of a, I can tell the bass is coming
from the left side of the room...although furniture vibration could be
giving it away too), but when I add music, I almost always
can't tell at all.  
     Now that you can hear those effects, imagine what kind of wave
interaction you'd get with *four* or more subwoofers at those low
frequencies!  It'd be a bloody nightmare.  You could probably
eventually get somewhat flat response at the listening position
(maybe depending on the room), but move your head and you could find
3 or 4 inch dead zones at some frequencies and peaks at the same
spaces at other frequencies.  Most likely, there'd be no good place
for anyone else to sit to get flat response over the entire bass
frequency range.
     So, while you're keeping an eye on various magazines, who will
almost definitely have differing opinions on what's best, keep that
little test in mind.  Remember, the speaker companies are probably
going to want to sell you more drivers more often than not if they
think they can get away with it.  After all, the more they sell, the
more money they make.  If you're really lucky, they'll value your
respect more than a few extra bucks on a one time sale, but don't
expect this from your mass market companies.  They'll probably say,
the more the merrier.  Stereo Review actually did some useful tests in
this regard for both AC-3, pro-logic, and subwoofers over the past
couple of years.  (amazing that Stereo Review could actually print
something helpful once in a blue moon...but even Stereophile, who
largely laughs at Stereo Review pointed the subwoofer article out as
good advice.  The AC-3 test was more recent.)
     To really find out though, you might take those home-built subs
you mentioned and place them in the back and run a sine wave stereo
test with your Legacy mains with low bass frequencies.  Then move
around the room a bit and see if there aren't large dead/peak zones
with all 4 speaker outputing bass around the 20-60Hz area at the same
time, if possible.
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