I think the main reason "loudness" was used is because our ears are less sensitive to certain frequencies at lower volumes to maintain what would be considered a "flat" presentation as according to the human ear which does not perceive linear sound linearly, but instead perceives dips and peaks heard at certain frequencies from this linear information. Even new equipment measuring as perfectly flat/ linear can benefit from loudness as according to the Fletcher-Munson curves. At lower levels, it makes music sound more realistic, satisfying, and full. This, despite the fact it's technically a deviation or departure from true linearity many strive to achieve with their sound systems. At the same time, a truly high-end system often does not leave people "wanting" more or something better with a slightly different, non-linear FR curve, and that's ok too. It really just depends on what people like and prefer.
There are other methods to achieve something similar as mentioned using tone controls or an equalizer. I use a mastering processor/eq and target better [general] linearity first (within the listening room itself whose irregular shape and wall reflections also cause deviation from true linearity to begin with). This is done through room correction using rta. Then, I apply a very slight Fletcher-Munson curve on top of that. There are many different ways of achieving something similar.
