EQ

You think the bass a bit to heavy, the treble to bright?
Use an equalizer to alter the frequency response.

You can do so according to your taste.

Today often the EQ is aimed at bringing the headphone in line with the Harman target curve.

Harman curve

The Harman curve is derived from double blind listening test.
Basically it is a response preferred by trained listeners.
They describe headphones having this frequency response as to have the most neutral, spectral balance.

The first findings where published in 2013 by Sean Olive.

As it is subjective, not everybody will appreciate.

Under the Harman curve, listeners are categorized into three potential groups:

Harman curve lovers: This group comprises people under 50 primarily and forms 64% of all listeners. They prefer headphones with sound quality tuned very close to the Harman curve.
More bass lovers: This group makes up 15% of all listeners and mainly constitutes male and young people. They prefer headphones tuned 3 to 6dB more bass than Harman curve at sounds below 300Hz, and 1dB more for outputs above 1 kHz.
Less bass is better: Females and mostly older than 50 form the most significant chunk of this group, constituting 21% of all listeners. They prefer bass below the Harman curve by 2 to 3dB and 1db more on outputs above 1 kHz.

Source: Audiosolace

AutoEQ

AutoEQ is a project for equalizing headphone frequency responses automatically and it achieves this by parsing frequency response measurements and producing a equalization settings which correct the headphone to a neutral sound. This project currently has almost 2000 headphones covered. AutoEQ is not just a collection of automatically produced headphone equalization settings but also a tool for equalizing headphones for yourself.

Sources are published on Github

EQ Settings for 700+ Headphones - Jaakko Pasanen