For headphone and speakers, different model have different frequency response, so you need to know what the curve looks like before you can use the eq to compensate for the response frequency that is quiet. Of course, after you adjust the response curve to relatively flat, there is still the whole loudness perception you have to take care of. It depends on your perceived level of loudness on different frequency (yes, we perceived loudness differently at different frequency even if you have no hearing damage so a flat response curve doesn't sound like equal loudness for all frequency for a person with normal hearing), if you factor in hearing damage (e.g. if I am slightly damage on my hearing from 20-60Hz, then I would really turn my bass up, but you would find the base too loud), there is no one size fits all for equalized .
Then there is the changes you make on the equaliser settings which depends on what you listen for. Some people prefer equal perceived loudness for all frequency (balanced listening) while others emphasize on certain frequency, usually depends on what type of music you listen to, e.g. if you listen to hip-hop, you emphasize on the base and if you listen to classical, you emphasize on the mid and high.
There is no standard answer for everyone. You will have to try the eq settings until you hit the right spot for your liking.