Honestly volume is like measurements - if you go by measurements, the Topping A90 would be the best amp in the world with totally flat response, no noise and high power. But many if not most more expensive amps are noisier and less flat by measurements, but many prefer them - maybe it's all placebo, maybe it's not - it's basically all subjective and how people feel about how something sounds.
Dbs are the same - depending on where in the frequency range the dbs are coming from, and whether it's pleasant or crappy noise, people perceive volume differently. So db measurement is a theoretical measure of sound pressure but is it like FR or noise measurements? Volume, loudness, like FR response and noise, are often a matter of opinion. Would I care if a db meter tells me two things are at the same volume while one sounds louder than the other because it's loud in certain frequencies that I am sensitive to? Honestly I care more about how my ears feel than what some meter displays.