Description of the Series 7 from the source:
Designed to mimic the unique overdriven tones of the rare Vox 4&7 series amplifiers in a small and easy to use stompbox. An unknown amp to most, the 4&7 series amps were most famously used by the Beatles during the Revolver era and Jimmy Page on numerous tracks from the second Zep album. Think Paperback Writer style of tones and, yes, Whole Lotta Love.
The 7 Series Overdrive captures the aggressive qualities of the solid state preamp found in the 4&7 series amps by using a pseudo hybrid setup consisting of a specifically selected silicon transistor for the first stage which adds a distinct bite and growl. That pushes an FET stage into clipping adding some warmth, sustain and amp like dynamics to the tone and feel. The end result is a highly effective emulation of the amp and a unique sonic blend that could be characterized as a "fuzzy overdrive."