Martijn Mensink from Dutch &Dutch explains how the 8Cs ‘bass couple’ sending one coherent wave to the listener.
‘The tweeter and midrange on the 8c's are acoustically aligned. They are both delayed to be aligned with the subs, so you get what is often called a virtual point-source. The two subs get the exact same signal. The bass works best when the 8c is placed relatively close to the front-wall. Then the wall and the speaker become a single system, with a hemispherical radiation pattern (this matches well with the cardioid radiation pattern above 100 hz).
When the 8c is placed right up against the wall, the 'reflection' against the wall is not really a reflection, it is perfectly coincident with the direct sound from the drivers. When there is some distance between the 8c and the wall, the acoustic center of the combination of the woofers and the wall (the point from which the sound appears to emanate) shifts a little towards the wall. With the presets we add a little more delay to the tweeter and midrange to again align them with the bass, so the result is still a virtual point-source with both flat amplitude and phase."
If you look at the near field measurements I took of the 8c's in the Objective Measurements section, one can see that indeed the direct and reflected sound are coincident. The result is a virtual point source with both flat amplitude and phase, as shown by my measurements.’