Is there a reference track that we could all use for this or is pink noise sufficient?
Would the use of an acoustic filter i.e. bandpass enclosure be a way of confirming that distortion is not the culprit?
I'd like to hear more on this hypothesis of yours and also your thoughts on whether this would be as pronounced in a car as in a room with the speakers some way away from the source of the reflections. There is a significant difference in terms of height in a car compared to the average room, so maybe the triangulation would be less of an issue - presumably we could test this in a room if we could use something to absorb and attenuate any reflected energy, or find a narrow room where the speakers can be placed very close to the walls whilst maintaining the same degree of separation from the listener - I actually thought about using the under stairs cupboard as a "car simulator" (narrow width, sloping wall to the front) but it's too full if junk right now to do this!
I'd say that you run into group delay issues with a bandpass, and the change in group delay may contribute or not at all, in enhancing all derivative distortion above the crossing frequency.
so, you deal with other problems at the same time as THD, and intermodulation distortion.
I personally don't experience much localization in a car below 300 hz, using test tones and well damped mounting locations.
this itself, flies in the face of convention and those golden ears who can localize 80 hz are possibly not aware of all the artifacts of a reproduction system contributing sounds above 80 hz, even if their crossover is believable and the system at lower volumes is highly defined.
I can probably attribute my lack of precision at 300 hz, to the average listening level I prefer to have my sound stage appear.
The louder it gets, the more the music takes on a "fill in all holes" quality, and the less particular I get over things like stage pull down, or to the rear.
and don't think of it as a neophyte's attempt at sound quality either, it's that same crosstalk that makes midbass palpable from both sides of the car, that also forces some destabilization of the image because of the higher distortion profiles of smaller diameter cones being made to play 300 hz and below.