It'd probably be possible to achieve a pretty flexible equaliser and general audio effects system using the pulseaudio LADSPA module. This would allow for any number of different LADSPA effects to be used system wide, e.g. reverb, echo, equalisation, pitch shifting, pitch correction, amplification, etc.

This module isn't installed by default, however I think it might be included in the pulseaudio-module-extra package. Unfortunately this is hosted in the repositories at http://harmattan-dev.nokia.com which have been down for a while now.

If Nokia ever fix the repositories I might have a go at writing something graphical based around this approach, until then its a bit too much effort.