Balancing the battery load often requires an equalizer
consisting of either a passive circuit, which can be inefficient, or an active circuit, which is far more expensive.
When using two or more cascaded resonators to realize an equalizer, the cascaded S parameters can be calculated as 
An equalizer using multilayered SIW technique is proposed in this paper.
You probably could use an SPL meter with an equalizer like the THX 44 if you had a test disc with discrete frequency signals at the indicated 1/3-octave intervals.
There is no doubt in my mind that employing an equalizer that can smooth out all three channels up front will highlight the advantages of any good three-front-channel sound reproduction, because it will allow the center speaker to better integrate with the left and right mains.
If you flatten out a power-response notch with an equalizer, even if you do a job that looks perfect when you check the results over with your real-time analyzer (RTA), you may end up boosting the narrow, forward-angle response of the system over that short frequency range.
The second problem an equalizer can deal with involves speaker/ boundary interactions.
Because this is a fully power-response related artifact, an equalizer can do a lot to correct the problem.
So, take care when using an equalizer to correct for these kinds of artifacts.
I find the predisposition of some individuals to smooth out power-response dips and peaks in the midrange by means of an equalizer to be both interesting and problematic.
Now, if a system does not apply such a cut automatically, as part of the dip/flair phenomenon, an equalizer can be used to deliver the same kind of effect -- almost.
Getting back to dips and equalizers, while this kind of response diddling with such a device can be interesting in larger rooms, it is in smaller rooms that the manipulation of midrange frequency response, either by an equalizer or because of the way the speaker systems themselves are designed in terms of their midrange response, can sometimes do wondrous things.
While correcting for room/speaker problems can often be a tedious process even with a good real-time analyzer to verify the results, attempting to use an equalizer to correct perceived deficiencies in every slightly sub-par recording that comes along will be an exercise in futility.
However, using an equalizer to make modest room/speaker corrections can often result in a good system sounding even better, but only if the system and the equalizer are able to get along together.
I will note that with the boost applied, the bass below 40 Hz was rolled off rapidly (remember, this is an equalizer control that is centered at 40 Hz, not a full-bass boost control), which will be a good thing with small speakers needing a reasonable amount of boost, but which would not be able to deal with strong boost at extremely low frequencies.