Stereo Subs - Do they need to be identical?

Shadorne, I think it depends upon the distance from the rear wall and the construction of that wall. Frequencies below about 45 hz or so pass through typical sheetrock walls and so no cancellation takes place. Therefore speakers more than 6 feet or so from this wall will not be affected by 1/4 wavelength cancellations. If the wall is reinforced then it is still O.K. if the speakers are more than 8 feet or so away as the loss will be attenuated by typical " room gain" below 35 hz or so. If 6 or 8 feet is not practical then try pushing the speakers very close to the rear wall and use thick absorption behind them. ( thick enough to absorb 125 hz) If this is not practical then yes nonalignment is a good idea. My bias is that the same frequency (crossover region) coming from two different sources at two different distances always creates phase issues and its own cancellation issues as well as slurred imaging in frequencies high enough to be directional. A lower crossover of course limits the damage of the last issue but not the first two. Each case is unique and the only way to know for sure is to experiment which in my experience takes a lot of time and patience and in any given case can lead to unique solutions. JMHO - Jim

Funnily enough the link I gave above came from a manufacturer...

The link also shows up to a 20 DB SPL hole in the bass in some cases due to quarter wave cancellation and a succession of harmonically related cancellations further up in frequency (a comb filter)...

Furthermore one might wonder why studios often soffit mount large full range speakers ?

These same studios often leave small (less than full range) near fields out far into the room on the meter bridge.

Perhaps there is some logic ....perhaps is is all just aesthetics...

Here is another speaker designer's comments on the reasons for soffit mounting speakers.

Read and draw your own conclusions. Dismiss the science if you like...

In some cases, the quarter-wave cancellation from the bounce off the rear wall can be taken advantage of to tame a peak.

As for asymmetrical placement, here's a link to a brief informal paper by one of the leading researchers in psychoacoustics, Earl Geddes. He's comparing four subs in corners vs four subs asymmetrical, so it's not exactly what's being discussed here, but might be worth taking a look at as it illustrates one of the advantages of asymmetrical placement. The thing to look at is how much the peaks and valleys are reduced for each of the three listening positions in the asymmetrical case (represented by different color lines):

http://www.gedlee.com/downloads/sub%20study%20.pdf

Duke

Here is the link to calculate the wall bounce from reflections. If you use stereo subs then you can assume the same thing will happen up to a couple of octaves above the crossover (when the effect starts to become negligible compared to the many other room effects/anomalies)

Yes science (theory) and experimentation are necessary to get you to where you need to go. Theory is a good starting place. Mounting into the wall (soffit) is an option and can be tried but you also have to factor into the equation the room length and how this placement will specifically fit into that. My point is that theory is just a starting point and that when you really get into this particular project you have to do your own listening and your own measurements because the variables in any given case are so numerous as to render any given prediction based soley on theory ( even very sound theory) woefully insufficient. - Jim