The thing is we can only notice the bass difference with combination of mid and treble. If we don't have the mid and treble in the spectrum to compare, we wouldn't even tell if the bass has been lifted, it will only appear to sound louder. Also when someone says "the bass on a earphone is so good", it doesn't mean the earphone sound great within 20 - 250 Hz, but in 20 - 200000 Hz. When we listen to drums, we are not only listen to the main body of the sound, we also listen to it's overtune, e.g. the air, the clarity, the directions, the resolutions and etc, which will require the integration of mid and high frequency ranges.
That was what I was trying to say actually, if we are changing the bass and trying to detect that change, should everything else remain constant (ceteris paribus)? I had thought that it was mainly the quantity that changes when we turn the knob. I figure the quality of the bass might change slightly too.
Suppose we turned the knob and kept the bass volume constant, to your ears, is there a radical change in the quality of the bass?