Is there an equalizer for mpd? Edit
mpd 0.13.0 can output to JACK which allows other tools to do EQ through it.
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.
A sometimes simpler solution is to setup Icecast as the audio output and pick up the stream with any standard media player that has an equalizer built-in.
It is also possible to configure the pipe output and use another application to process the audio. For example, ecasound can be used together with ladspa plugins to provide equalizer and some other features.