[I'm not 100% sure I understand your question, however I will assume I do understand and will now respond.]
Before the advent of Audyssey and other microphone/computer assisted room calibration schemes, placing two identical subwoofers the same distance from the primary listening position, and setting them to the same xover and level, let's say my favorite one, "11"
, was a reasonable course of action.
The problems with this, which Audyssey owners can have fixed for free, are as follows:
A. What if the two subs aren't the same make and model?
B. Even if they are the same and they are both set to "11", the actual level arriving at the seated primary position may vary due to the differences between the two subs' proximities to room boundaries (distances to walls, ceiling, floor) or room decor and reflections differences.
Generally, all other things being equal, introducing another identical speaker to a room (sub or
full range, it doesn't matter), backed with the same signal and symmetrically identical placement, will yield a +3 dB boost to the room. [Which is nice, so you can then run them at a lower level with less woofer excursion and distortion, plus it can get louder when it needs to.] This however flies out the window if one speaker has a 3 dB (or greater) louder output than the other one, for whatever reason (volume knob settings, different efficiency designs, room boundary distance discrepancies, etc). Suddenly that +3 dB room boost vanishes and the 2nd speaker's contribution to the room becomes practically nothing
This is why it is important that a secondary sub added to a room be set so that its volume, as measured at the listening position
, is the same as the first sub. If either sub is 3 dB (or more) louder than the other, then the weaker one has almost no
consequence on the room's bass, as far as that listener is concerned. You might as well not have even bought that second sub!
Although not nearly
as critical as it is with the other speakers in the room (and for different reasons), two or more subs in a room generally (there can be exceptions*) should be placed equi-distant to the primary listening position. It has nothing to do with imaging and spatial cues, however (despite what some "audiophiles" might insist), but instead is simply because you don't want the subs out of phase and working against
each other; it's just wasteful (and can cause frequency response irregularities due to constructive/destructive interference similar to "lobbing"). XT32 has an advantage over XT in being able to do that phase correction ("distance") with an electrical
delay, instead of my method I outlined which uses a physical, travel time delay.
*Although we AV7005 owners have a fantastic EQ which can correct great errors in the room, set via a computer controlled microphone feedback system on board, an argument can be made that one doesn't want to excite the room identically with two subs having symmetrical room placements, ie room frequency responses due to room nodes. It is off topic to this thread's discussion however.
why wouldn't I run Audyssey with both subs on?
Once you have ensured they are both playing at the same level, as measured
from the primary listening position, and are reasonably in phase with eachother [I wouldn't worry about small diferences, say under a foot or so, considering the wavelengths involved] you do
run Audyssey with both on. It's the last step, "G".