Chu Moy has posted a design for a pocket EQ on headwize.com. He says in the article: "Koss Corporation makes an inexpensive, small 3-band equalizer for headphones that runs off 2-AAA batteries. Some portable stereos have built-in equalizers. These options may not be satisfactory for the serious listener, since they are not designed with audiophile concerns in mind."
Superbaldguy, from what I've read the Koss EQ-50 is crappy. The EQ-50 suffers from the very same faults as its predecessor, the EQ-30: a high noise floor and very little headroom (you barely boost any of the three bands, and then - WHAM! - clipping and other horrible kinds of distortion set in).