Headphone preference tolerances are tightest in the speech-critical midrange.

Evidence

What is remarkable is that when the curves are normalized at 500 Hz, the preference tolerance are quite narrow between 300 Hz to 2 kHz, the most important range for speech and music. The bass and treble regions are still quite narrow, ±2 dB below 125 Hz, and above 8 kHz with the broadest tolerances ±3.5 dB falling between 3 and 5 kHz. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1849

Categorization

• QUESTION: The broadest tolerance (±3.5 dB at 3-5 kHz) sits right in a highly audible region — worth chasing whether listeners are simply least sensitive there or whether individual taste diverges most there.
• XREF: Echoes earlier chapters’ findings on room-response/target tolerances and the primacy of the midrange for speech and music perception.