The EP-PRE is a JFET preamplifier that is based on the preamp section of the Echoplex® EP-3® tape echo unit. It will deliver a few db of boost, but I don't consider it as a boost.
Words such as Enhance, Fatten and Enrich all cover the effect that the EP-PRE has on your tone, but I have yet to find a word that really captures the magic of this amazing little circuit. A friend who has one of the prototypes calls it a “Tone Polisher”.
My own feeling is that it creates a “focus” in my tone and helps me to play with more confidence.