That's nonsense. The purpose of anechoic chamber measurements is to take the real world out of the equation; to measure what the device is capable of, sans any effects introduced by boundaries. In fact, you'd find it pretty disturbing to listen to anything in one.
I didn’t say anechoic chamber… but I can see how you would make the leap to that given what I did say.
What I said amounts to, even if you have the means to discern “coloration†from direct, it will be for naught unless you have a means to control or otherwise influence the “coloration  and I know it will require more than any listening test you can devise.
No. Flat response is flat response. The 'C' weighted measurements, or more accurately, the 'A' weighted measurements (the one based on Fletcher/Munson curve used in the 'loudness control' you mention below) were designed to make up for the difference in the perceived loudness of sine waves versus the full spectrum of noise the ear and brain evaluate.
You are correct on the weighting scale. My bad. I haven’t discussed weighting scales for a while now, and my last occurrence was regarding occupational noise levels and hearing protection. Many of the meters used to determine occupational noise level are C-weighted.
As far as curves are concerned, I was more so referring to the Robinson–Dadson model, as revised per ISO 226: 2003.
I have not heard or read anything that states the curve is different for sine waves than for noise (e.g. pink moise). Got a link to an online source which confirms that assertion?
Again, flat response is flat response. While you could introduce a loudness contour for any set level, that would only mean all other levels would be wrong as I have yet to see a 'loudness' setting on any gear that actually track level and adjust the curve according to the ears' diminished sensitivity to deep bass and high highs at lower levels. More often than not these contours result in booming bass and piercing highs. If you like it that way, fine (I hear plenty of vehicles driving by with that kind of response curve), but it ain't accurate. And my hearing deficiencies may be different then the sampling done by Fletcher and Munson 50 years ago; does that mean I need to account for those as well? None of these curves are applied when listening to live music, so what is the point? Get it right first. If you then want more bass and treble at lower levels, use tone controls or EQ.
The point is you/we are not listening to live music. Your paradigm is fine if recorded music were not EQ’d, compressed, ducked, etc. The processed electronic signal is far from flat as the original audio energy wave was. Believe me… I used to mix live sound and do multi-channel recordings for demo’s for several bands in my area. (I don’t do it anymore because my “day job†leaves me unavailable too often.)
The concept of loudness control represents the ideal. Implementation typically falls short, but some are better than others. Knowing how to set up a system for optimized contour is key in most scenarios, but the manufacturers that include a contouring feature never provide details for proper set up. But you are correct in that some to many implementations will just be outright intolerable to a refined ear.
And your assessment of the cars driving by may be inaccurate. The quality, if set up for such, will be inside the car. What you hear on the outside can be a world apart. But I do agree many are boomy… but I believe that mostly stems from people liking it that way… and not having any clue what accurate reproduction is all about.
Perhaps I should have been more concise in my original response to your suggestion to the OP that he listen to sine wave sweeps to adjust parameters based on perceived relative loudness. It won't work, and he'd be wasting his time.
To each his own… but that seems a bit contradictory with many system setup app’s using a combination of swept sine wave and pink noise RTA measurements.
FWIW, I have also been one-step short of using active acoustic correction on my home theater setup. The thought crossed my mind before I actually had it setup up without. Sounded so good without I dropped the thought. Perhaps one day I will follow through...
In the meantime, nothing you have stated changes my position…