I was looking for a way to add an equalizer to QuickTime Player and Cog Music Player on my Mac and although there is no specific add-on that you can use, there are several software packages available that add volume boost and equalizer features. And they actually work across your entire system, so they are not limited to any one particular piece of software.

The oneI found that is easiest to use is called Boom by Global Delight. It gives you basic volume boost control with hot-key access, as well as a 10-band equalizer with many presets. It places an icon in the menu bar at the top so you can access the EQ quickly.

There is a 7-day free trial available on their website, and it is also available for purchase from the Mac AppStore.
http://www.globaldelight.com/boom/

Let me know what you think of it by leaving a comment below!