> Basically, the sub question comes down to a few things.
 > 
 > If you have properly optimized the position of your main speakers and
 > found the most even bass response, then you have also found the
 > proper place for your subs.
     I disagree.  When you get down to 40Hz and below, the average
room response for a typical size room will not be conducive to a flat,
even bass across much of the listening area.  Notice, I don't mean
*one* seat in the sweet spot.  I already said one might achieve flat
response for one person with some experimentation.  But the issue at
hand was AC-3 and AC-3 implies movies and movies often imply more than
one person watching and listening.  You will almost never get the best
overall bass response in terms of off-axis response by placing your
subs (or even the woofers on the main speakers apply) where the
midrange and treble sound the best for stereo imaging.  
 > There is only one (well, maybe a few) good spot for your main speakers,
 > were the bass will be even and as proper as possible, when auditioned
 > from the listening position. The standard mucho experimonto disclaimer
 > applies here.
     
     I note you say "listening position" singular.  Are you only
considering the sweet spot seat?
 
 > Your subs will be happiest in the same position, if you have done your
 > "homework" properly.
     I seem to recall some heated arguement over Stereophile magazine
and their opinions a while back between Lewis, you, and me.  I don't
wish to stir up such a thing again, but I do suggest you do your
homework as well.  I wrote my advice to John before I read the this
month's Audio magazine and was pleased to see an article in this
month's issue that addresses this very topic. I noted that even though
the writer went into his test thinking much like you seem to about stereo
bass and subwoofers with AC-3, that his tune changed as he went on and
his final advice after the tests were done almost matched mine
exactly.  I suggest you read the article.
 > Stereo Bass DOES work, and for all the right reasons. Pressure wave and
 > response patterns dictate that -when properly set up- stereo subs, in
 > the same
 > location as your mains, will sound the most believable.
     There is only one *best* place for bass below 40Hz and that's in 
*one* corner.  Use two subs if you need to, but place them in the same
corner or you *will* get interference patterns.  Have more than 1
listening position and you won't get the *best* results that way.
And even though you say that human hearing's limitations below 80Hz
aren't the real issue, the fact is it exists and is there to either
ignore and pay the consequences or heed and reap the benefits.  
 > Few subwoofers are actually that, and almost all will venture into
 > the range of audibility, as far as directionality cues go.
Well, if you call 36dB down in that area, yes.
 > The big deal is the pressure response
 > pattern that is recorded by the microphone.
     You're forgetting two things:
          1: Room response in the playback environment
          2: Interference patterns over multiple seats at low freq.
     Both of these will skew your expected results.  Once again, the
Audio article (and an older Stereo Review test covering stereo
subwoofers) describes and explains the problem.
 > That it is a low frequency, and our localization capabilities are not
 > really
 > existent is not the issue. It is the pressure pattern set up by the
     It's an issue in a real room with multiple listeners and real
physics.
 > usage of
 > TWO microphones in a stereo recording. If the recording was done wholly
 > in a stereo -wideband- micing technique, then this info comes shining
 > through. To
     I have discs with *low* bass (below 20Hz)
(Mark Weiss will attest to that) that are indeed full range stereo and
they do indeed image like normal stereo even though I have only 1 sub
in the corner and cross straight to my ribbons.  Why?  Because the
directional cues are all at higher frequencies and are covered by the
main speakers in the conventional locations.  I also get even bass
response across the entire listening area (a 3 seat couch and a
recliner) within 2dB of each other.  There are no interference
effects and standing waves are minimized.
 > duplicate the loading effect at the listening position, then two subs
 > are required, and in proper phase with the main output.
     You just won't hear it below 40Hz and very rarely below 80Hz.
What I don't hear, I don't worry about.  What I do worry about are
very *real* room effects and interference patterns at low frequencies.
 > I am sure that John Allen, with his very wide band (near 20hz response,
 > I think) Legacy speakers can tell you of this effect.
     He can also measure off axis of the sweet spot and verify the
peaks and dips caused by it as well.  Throw in two more subs for the
surround and really watch the the interference patterns go at it.
 > Back to the AC-3 thingie.....
     Wow, we finally get back to what I was addressing all along.
 >  Sub placement just don't matter (well sorta)
     Well sorta?  I'm not buying my stereo from you! 
 
 > You can critically place the sub using a test disc, tones, music,
 > etc... but as soon as the scene on the screen changes, the
 > micing is generally shot,
     What if you're watching a live AC-3 U2 concert?  (yes, there is
one).  Is the micing shot then?  The point is to avoid the room
problems.  Avoid them and the soundtrack and music discs take care of
themselves.  (assuming your speakers and the rest of the equipment
are of good quality).  I don't care how well miced a concert is, if
you have standing waves and peaks and dips in your listening room at
listening locations (note plural), you're going to be distorting that
recording in your room.  Why limit your playback to such distortions
after you've just spend several thousand on your hi-end gear?
Especially if all it requires is putting the subwoofer in a corner and
matching the crossover slopes so that bass below 40Hz (or even 80Hz)
is not sent to the main speakers?  
     Once again, I'm not making this stuff up as I go along.  (funny,
I think I said something similar the last time....)  I based
my original message on several articles I've read over the years (even
Stereophile agreed with this advice when it comes to subs) and
my own personal experience with a single subwoofer in a corner
(Keep in mind my AL-IIIs have bass response to about 30Hz and strong
output at 40Hz.  I've disconnected the woofers because I get better
bass without them.  There *were* interference patterns and I would
have to switch to active crossovers on the AL-IIIs and limit their
low frequency range for them to serve any useful function and then
only to lower the sub's crossover less than half an octave for some
very minor improvement in "tightness" over a small region.)   Finally,
you can read an article confirming everything I've said in this
month's AUDIO magazine.  I can probably dig up the Stereo Review and
Stereophile magazine articles if you really want to read them too.
     However, if "stereo" or even "6 subs for AC-3" works best for
you, then by all means, I wouldn't want to try and stop you from
spending all that extra dough.
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... "What a depressingly stupid machine," said Marvin and trudged away.
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