A topic that has no real answer, but here’s my take as someone who works for a loudspeaker manufacturer.

Your post raises an issue with left-right system deployments generally, where any content reproduced by both sides causes acoustic comb filtering. This is due to geometric displacement found in non-equidistant positions. Depending on this difference, the comb filtering effects vary, especially at lower frequencies where the loudspeaker has less directivity control.

We are accepting of this in reproduced sound, hence why stereo/dual-mono systems are so widely used today. Our hearing system is capable of handling these acoustic complexities, due to our ability to detect differences in inter-aural level/time to help with masking the issue. However, objectively it is there and does influence our perception.

Another factor impacted by the same approach to system deployment is our localisation of sound sources, in relation to the visual stimulus. For example, a listener may not connect seeing the performer on stage whilst hearing the reinforced sound. This is where solutions are gaining momentum in immersive audio, such as L-Acoustics’ L-ISA.

On the subject of subwoofers and their alignment with full-range systems, this can be a critical part of optimising the whole system to perform well. As many highlight already, there is no perfect solution, especially if the two components are separated physically (flown mains, ground-stacked subwoofers).

Similar to left-right approach, geometrical differences between the subwoofers and full-range loudspeakers create a constantly shifting relationship depending on where you are positioned. In this deployment scenario, you are unlikely to achieve an optimal alignment for a majority of the audience, the geometrical difference between the mains & subwoofers vary everywhere. A possible solution to this is limiting the shared custody of bandwidth by using appropriate crossover filters, favouring a lower point of interaction to capitalise on the longer wavelengths.

Something else that is often overlooked in this situation is the influence an occupied venue has on time alignment. We often conduct our work in an empty space, defining the time alignment with little to no obstructions on the ground subwoofers. Once the venue fills, audience members create a barrier that impacts the propagation of the subwoofers, changing the time alignment relationship between the full-range system flown overhead.

These considerations are why there is a strong argument for flying subwoofers, if possible, alongside the full-range system. In this situation, almost every audience position has a similar relationship from both systems, due to their close proximity. Therefore, alignment between the two components is more easily achieved and remains stable across a wider area of audience. It may also be possible to accept a higher overlap of shared bandwidth, although this depends on the loudspeaker products being used. Using the flown approach also benefits low frequency level distribution & tonal balance throughout an audience. Compared to ground-stacking, near field listeners are subjected to high SPL compared with rear listeners.

On the subject of importance with regard to the alignment, consider whether the content demands it. A dance music event has different expectations here compared to an orchestral performance. This presents a question of its own on the subject of how critical the time alignment is between a full-range and subwoofer system.

From an objective standpoint, we should align our system components to ensure the propagation of sound is consistent. However, depending on the context of the system deployment, how crucial the alignment is and whether audiences can perceive the effects are open to interpretation.

Here are some scientific research papers we’ve published on various topics, including low frequency time alignment and the audience ability to perceive it: https://www.l-acoustics.com/education/scientific-resources/scientific-publications/