DFX is a small DSP/Effect plugin developed to improve audio quality in Winamp. Its interface is really small and easy to use, it has slide bars to control fidelity, ambience, tridimensional surround (only available in plus version), dynamic boost, and Hyperbass.
You can change processing modes from a group of presets that include Large ambience, Concert Hall Ambience, Big Bass, Hip Hop, Techno, 50s and 60s rock, among others. (Only light processing is available in the free demo).
There is an option that switches between listening environments: speakers and headphones, but it is only available in the Plus version.
I am not really sure if I would pay 30 dollars for a Winamp plugin but If you agree with the price you will probably enjoy it. It has a fast response time, and the difference in audio quality can be easily heard. There aren't many additional tools, but this application works really good for its purpose.
The first time I run it, I had some problems with my system, Winamp collapsed and had to be closed. As a Windows user I am used to that, but you should probably try it for yourself to avoid any future problems.