I think you’re mixing apples and oranges.
Higher sensitivity speakers will produce a higher sound level with the same power input as a lower sensitivity speaker.
Loudness functions adjust the EQ.
From Sweetwater.com: Fletcher and Munson were researchers in the ’30s who first accurately measured and published a set of curves showing the human ear’s sensitivity to loudness verses frequency. They conclusively demonstrated that human hearing is extremely dependent upon loudness. The curves show the ear to be most sensitive to sounds in the 3kHz to 4kHz area. This means sounds above and below 3–4kHz must be louder in order to be heard just as loud. For this reason, the Fletcher-Munson curves are referred to as “equal loudness contours.” They represent a family of curves from “just heard,” (0dB SPL) all the way to “harmfully loud” (130dB SPL), usually plotted in 10dB loudness increments. Though the Fletcher-Munson Curves are by far the most widely known contours, there have been others defined in recent years that some engineers think are more accurate.