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RMS of course..

Specs give you a rough idea where things are at, but unless you know the extact specs of your senses and how it correlates to what stimulates them, it means very little.

The human response is irrelevant in this case -

is the first one.
I think that's how the old adage goes.

If you want to hear it the way the recorder intended.

Just gets me to thinking of how the TV makers set up video with exaggerated colors because people like it that way instead of actual color renditioning.

I just thought for a minute seeing that graph that I wasted my time buying speakers which are supposed to have a flat response.

The chart reconfirms what's been well known for decades ... the low frequency sensitivity of the human ear falls off dramatically at lower sound levels. This was the reason for the "Loudness" button on receivers, a practice largely abandoned today.

I did find myself adding four custom low frequency boost contours to my Velodyne sub, and I do use them at lower listening levels.

but one article I read about a decade ago talked about getting the wax out of ones ears will raise ones hearing by 10db

I always preferred just turning up the volume.

The Japanese manufacturers used to give it their all (within budget) to maintain as flat a response as possible with their sources and amplification. The result? A rather boring one.

Tube amplifiers don't measure anywhere near as well as many of the more clinical solid state amplifiers, and yet many people who spend huge wads of cash of audio gear following extensive auditioning, much prefer the sound of tube amplifiers.
Same with vinyl rigs compared with CD players (the latter being the less desirable of the two to those who do the comparisons). It's not all about how accurately the products measure, it's how they gell with each other, the room and your ears.

It's like if you find a girl who is similarly attractive as you (say both of you are an 8 out of 10 for your respective genders) and you both have very much the same interests, tastes, beliefs, political leanings etc. Doesn't mean you're going to get along.

Thanks to the effect of amplifiers, crossovers, over-lapping information from more than one drive unit, a panel's inability to cover the entire frequency range etc..
Which is why the best system any of us have ever heard, still isn't able to do some things others systems can do.

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It wasn't just that it increased the amount of db's I could take in, but certain sounds were a lot more interesting after the fact. High frequencies in particular. Violins sounded amazing, but nothing more so than the sound of a blackbird outside my window.

All bets are off when you play a recording at any other level, higher or lower. As long as you use the volume control on your preamp or receiver, you have a legitimate reason for non-linear response.

Of course it depends on the amplifier, the speaker being used, the volume level desired and the size of the room.