The VOP9 adds natural and warm nuances as would a good vintage tube amp. It combines a clean boost and overdrive and, when the Drive setting is at minimum, the overdrive circuit is decreased by 12dB whereas the boost is increased by 6dB. As the Drive setting increases, the overdrive gradually takes over the clean boost. It uses the same overdrive circuitry as the OD9. An internal switch allows to boost the input voltage from 9V to 18V in order to eliminate involuntary distortion that can create high output or passive pickups.