This, straight up, does not work. I opened Spotify and turned on some music and there is no difference. I even tried refreshing, saving presets, using built in presets, etc. Just doesn't do anything at all.
I have been trying to find a good equalizer for my mac. Sadly, they either don't work or are expensive for how simple an equalizer software is. It really should be built into the OS, especially considering that Apple created the iPod and how connected they are to the music industry.

I needed something like this bad.
I have some bluetooth headset that has horrible treble, this addon seems to reduce it a lot. Hearing crackles in audio off youtube though not sure why

SURPRISING !!!

Doesn't work with Napster (streaming service) out of the box, unfortunately. Although neither does "Audio Equalizer and Amplifier," so maybe it's just Firefox not playing nice with my system.

Indeed works with HTML5 audio but unfortunately this doesn't work with music/radio extensions.