EQ10Q appears to be the absolute best quality equalizer available for Linux. I have tried that that comes with pulse-effects, but found that it introduced occasional audio glitches; no good for audiophile use.

EQ10Q is an LV2 plugin, so I start from the assumption that you already have JACK installed (doing so is trivial so I will not include instructions here). The following set of steps are what I used to get EQ10Q working:

  1. Emerge dev-cpp/gtkmm, since eq10q depends upon it for it’s UI.
  2. Download and install eq10q (do not use the ebuild available in the darkelf repo).
  3. Attach the audio-overlay repo (layman -a audio-overlay).
  4. Emerge media-sound/carla (requires accepting the live ebuild in package.use, plus enable the gtk2 flag because the eq10q UI uses GTK2).
  5. Run carla and add /usr/local/lib/lv2 to where carla looks for LV2 plugins.