Passive EQ is cut-only, with the exception that there may be resonant peaks at some settings. The advantage is chiefly the tonal quality of the inductors and other premium passive components that are used in "boutique" examples like in the Aggie head. The components have a different sound than a typical active EQ. Additionally, the shape and interactivity of the EQ profile from a passive setup is different from most active EQ's, and some people really like the way that works, with kind of an organic, intuitive shaping rather than "boost 500 Hz by 3 dB".