In Gentoo, all you need to do to get this working is to install pulseaudio with
equalizer USE-flag enabled (may need to unmask the unstable ebuild) and append this line to
~/.pulse/default.pa (you can change the sink name to your liking) :
load-module module-equalizer-sink sink_name=equalized
After that, quit all applications that use pulseadio and restart pulseaudio:
pulseaudio --kill && pulseaudio --start
Now you should be able to use the equalized sink name for your multimedia application. For example, in Clementine, go to Tools » Preferences » Playback and change the Output plugin to PulseAudio Sink and fill the Output device field with your equalized sink name (
equalized in the above example).
While Clementine’s playing, adjust the equalizer levels by running
qpaeq. All done!