Obviously Dave has wayyy more experience with car audio than do I, but perhaps I can provide some of my personal thoughts on the use of equalization devices.

Like Dave said, many professional installers will honestly state they do not condone using EQ's or similar equipment in their own installations. This is primarily because you are introducing another device into the signal path that can degrade or otherwise alter the system's sound in subtle & not-so-subtle ways that were never intended.
In other words, the simpler & more direct the signal path, the better. I am not an audio/electrical engineer so I can't explain the technical details of why this is true, but suffice to say this is the rule of thumb. Perhaps Dave can go into more detail.

Also, the belief of installers to avoid using such devices is due to the fact the head unit should provide enough adjustability (bass, treble, mid range, etc.) to satisfy just about anyone. Adding another device that allows even more adjustments can result in horribly unnatural music reproduction due to the potential for someone who is unfamiliar with an EQ's operation & limitations to become confused.

Likewise a competent professional installer will be certain that the basic component selection & location of the various speakers adequately compensate for any sonic "holes" or other deficiencies in a particular vehicle's interior that an EQ or similar device is intended to remedy via its adjustability.
Of course not everyone can afford or would necessarily want to have their vehicle interior completely rebuilt so that all of the speakers will be installed in their ideal locations, therefore the popularity of EQ's & similar components.

Such is the case with the system in my SVT. Originally I did not have the Eclipse 2101 parametric equalizer installed as I thought the adjustments on the Nakamichi CD-40z head unit would be sufficient. However, after several months of highly critical listening & making adjustments almost continuously in an attempt to tailor the sound to my preference, I ultimately decided to add the EQ. Now I am completely satisfied with the overall balance in my car.
Although I must admit to being a bit curious about the Audio Control HPX Spatial Expander device, if only to see how it affects the sound compared to just the EQ.

I guess I'm just a hopeless gadget twiddler... :rolleyes:

1999 SVT Contour, #2140 of 2760, Tropic Green - Medium Prairie Tan
Koni Sport struts; TSW Blade wheels; Nakamichi, a/d/s/, Boston Acoustics, Infinity I.C.E.

1989 Saleen Mustang, #89-0408, too many mods to list here