Set a narrow "Peak" filter. Boost it by +10dB. Sweep the frequency dial from 20Hz to 16kHz. Where does it sound loudest? Where does it hurt? Note those frequencies.

Before we list specific numbers, we must understand the "Why." Humans hear frequencies differently based on volume (Fletcher-Munson curves). A flat response is great for studio monitors, but on earbuds or car aux systems, flat often sounds "boring."