Louder sounds better. Not metaphorically — a difference of half a decibel is enough to make people reliably prefer one thing over another and describe the louder one as clearer, fuller and more detailed. Almost every “this amplifier is more transparent” impression is this effect wearing a costume.
If you want to compare two headphones or two sources honestly, match the levels first. This works out by how much.
The number above is the difference in voltage sensitivity, which is what decides loudness when both are fed the same signal. Headphone sensitivity is usually published per milliwatt, and per milliwatt is the wrong unit for this question — an amplifier delivers volts, and two headphones of different impedance turn the same volts into very different levels. The converter explains why that one unit swap causes most of the confusion in this hobby.
To apply it: use a fixed digital cut of the stated size on the louder one, or a measured number of clicks on a stepped attenuator. Do not do it by ear — matching by ear is the thing you are trying to eliminate.
If you have any way to measure, a cheap sound level meter or a phone app against a coupler will get you inside a decibel, which is enough. If you have neither, matching by the calculated figure is still far better than not matching.
Switch quickly. Auditory memory for timbre decays in seconds, so a comparison that involves unplugging cables and refitting earpads is measuring your memory rather than the equipment. Use the same passage. And if you can arrange for someone else to do the switching without telling you which is which, do — not because you are gullible, but because nobody is exempt.
This is not an argument that everything sounds the same. Headphones differ enormously and you can hear it instantly. It is an argument that sources mostly do not, and that the honest way to find out is to remove the two variables — level and expectation — that reliably manufacture differences which are not there.