Does anyone here think that crossing over to the ATC mid at 380Hz (like it is done in the ATC line) is a bit too low and creates distortion / compression at high SPL levels?
The harmonic distortions (or IMD) is certainly not a problem.
I have no problem with your persistence. While looking at the Stereoplay pictures above, on the right side (active ATC), I can see 60 degrees loss is less than 10db at 10khz and 15db at 20khz. To me that looks rather good.
The horizontally asymmetrical arrangement of the midrange and tweeter is a problem if you value the most even radiation possible.
The reflections that reach your ear should have a frequency response as similar as possible to that on axis. This prevents the brain from classifying the incoming signals as different sound sources and prevents the sound character of the LS from changing at high sound pressure (when the diffuse sound has a greater influence).
Of course, the absorption behavior of the listening room plays a major role here - the speaker manufacturer cannot influence this. But he can ensure that the loudspeaker is as uniform as possible in its sound radiation.
The asymmetrical arrangement of the drivers on a baffle with "hard edges" has an advantageous effect on the frequency response on axis (if optimally arranged), because the interference of the edge diffraction will then be balanced out to a certain degree.
Due to the asymmetrical arrangement, the propagation time to the side edges is different and thus the horizontal angle measurements for the right and left side will be different.
As an example, let's look at a listening room with the first order reflections (reflections with one reflection point) of a loudspeaker to the listener. The loudspeaker is parallel to the side wall:
Now it would be desirable that the frequency responses of the reflections correspond as much as possible to those of the direct sound (in our example the 20° FR).
For this we look at the horizontal frequency response measurements of the loudspeaker. Here are the simulated frequency responses 0°, 10°, 20°...180° of a loudspeaker with a large, symmetrically arranged waveguide.
In the diagram, the 20° FR has been normalized to better compare how much the reflections at 40° and 60° differ from it.
The 40° FR hardly differs from the 20° FR. The 60° FR drops evenly to high frequencies, compared to the 20° FR.
There are no sudden rises or crossing FRs.
The whole thing again in the spectrogram representation, there the individual FR are less well to be recognized, but for it one can recognize that the loudspeaker has very evenly directivity.
To be able to assess this, you need comprehensive measurements of the LS, which are unfortunately not available for the ATC SC150.
Therefore, it is difficult to estimate how much the asymmetrical arrangement of the drivers affects the directivity of the LS - of course, only if the directivity of an LS is important for you
Using the limited measurements (in a horizontal direction) of the ATC SM50 from
post#179, the following graph results if normalized to the axis frequency response:
That's not bad so far, but it could be better
UPDATE: In reality, everything is much more complicated and the explanations I have given are very simplified.....