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Good arguments all Phusis. I can think of locations we’ve been involved in where symmetry worked; other places it did not. Visually we are always drawn to symmetry. My clients want the subs right under the mains and this "looks right". But most of the acousticians I know would argue against it, even if the physical offset is smaller than they are asking for. Their argument is that both subs at 12 feet from a corner sets up a mode relating to 12 feet in length. One at 12.5 and one at 11 is better. 4 subs should be at 4 different lengths from corners. An alternative plan is an array of all 4 subs next to each other on the front wall but we can leave that for another time.

One difference may be that in my applications, the mains are NOT high passed. The argument is that all crossovers create a hole (a dip at the crossover point itself) which is increased with increasing slope of the crossover. In studios, a 48dB slope is a big NO. Most of our applications have a 12dB slope, minimizing that dip. Introducing a dip right at 100 or 80 or even 60 is very noticeable and counter intuitve to the goal of "more bass". The other issue is phase and the introduction of another phase shift that is not wanted. Subs are low passed, usually with gentle slopes.  

In all types of music now, we see extraordinary low end being inserted at the artist behest. London Grammar’s "Hey Now" is adjust such an example of unexpected but likable low end.

In our business, a filter to make sure a speaker rolls off at a new higher place is not desired, as now we have added something that makes a sound across the mains and changes them.  Even in the case of a simple and well executed LF filter, it is audible. In this application, low passing subs and not filtering anything on the mains is the basic idea.  Subs are usually sealed, have no EQ and no DSP of any kind. Blending them can be a relatively easy process and sometimes requires inverted polarity (180) to make work.  Depending on arotating phase control, which only works over a portion of the sub band, is not the fix but a fine tune. 

In large rooms one wishes for DSP to create a delay of the mains to match up to the subs, but this is often not accepted due to the audible change the DSP introduces plus the barely noticeable delay created by DSP. Any offset in time makes tracking odd, sometimes difficult where one artist is in the live room and one is the control room- both being recorded at the same time.  If the entire system is DSP driven that may be a different scenario, but with ATC we prefer 100% analog output to speaker.

Brad

Sorry @lonemountain but high passing mains,. especially residential is often the right thing to do, and done right, with quality subs and filtering on both is inaudible. There will be no shift in tone and no localization is possible.  You have to be cutting off below the Schrodinger frequency. Your subs have to be low distortion, especially anything third order and higher. That may be as much a consideration for where you're cut off frequency is as anything.   If you've got large means that can play low distortion at high volumes at base frequencies, then I would consider it beneficial to run them full range. But most mains these days don't fit that criteria. For most means I would far prefer a cut off that's not full range especially if the customer likes high volume levels. 

I don't understand your comment with respect to DSP. With any modern half decent quality ADC, you're not going to hear any sound of using an ADC and then DSP. And of course if it's an all digital system, it's a nonstarter. Talking about DSP delay is also not relevant when you bring up in the same paragraph 180° out of phase operation which is a delay. There are no absolutes speakers like a line array can be difficult to integrate with subs and an evaluation needs to be done based on mains speakers, room size, listening levels etc

@kota1 phasing sub and full range speakers is not difficult, just put you head exactly between the sub and the speakers, play and 80hz tone with all your equipment on and slowly turn the phase knob until you hear the null point and the strongest point. Go back and forth to be sure you are not 180 degrees off, stop at the strongest tone. 

thanks

@lonemountain , I beg to differ Brad. Subs belong in corners. That is where they are most efficient. I have been using digital signal processing including full range room control for 25 years. My main speakers are ESLs and I high pass them at 100 Hz. The result is far less distortion and much higher volumes. The advantage will not be as great for multiway dynamic speakers but still detectible. IMHO digital signal processing done right is a boon to overall system performance for a multitude of reasons not just subwoofer crossover. You can not perfectly match your channels any other way not to mention tuning your system exactly the way you want it. I even use dynamic loudness filters. The system sounds exactly the same regardless of volume.

The best way, IMHO, to manage subs is to put them where they are most efficient using at least two in a symmetrical array with the main speakers, cross them steeply (48 dB/oct at least) using matching high pass filters on the mains putting the crossover point 6 dB down. Then matching the subs in time and phase with the main speakers. I would argue that if a symmetrical array can not be done due to room issues then it is the wrong room for a HiFi system. I have set up systems in rooms I would have never used for HiFi and made the best of it but I always warned the person the situation was not optimal. Most people do not care. They listen to music but do not imagine it. To them an image is something coming from the right and something else coming from the left.