From Music Player Daemon Community Wiki

[edit] Is there an equalizer for mpd?

mpd 0.13.0 can output to JACK which allows other tools to do EQ through it.

JackEQ and JAMin have been tested and is capable of providing EQ for mpd, but there are probably more JACK applications out there capable of providing such functionality.

When using JACK with these equalizers, be sure the default connection from mpd to the underlying sound device (usually ALSA on Linux) is disabled. mpd should instead be configured to send audio to the equalizer application; and the equalizer application should then output to the sound device.

qjackctl is an easy-to-use interface for configuring and managing JACK connections.