The SM9Pro+ was developed based on the distortion core of the Maxon SM9 (Super Metal 9), which was used a lot in the late 1970s. It creates scooped heavy metal tones with high-level gain circuitry and cut midrange circuitry. It operates under 9V but an internal switch can be set at 18V, increasing dynamic headroom, stabilizing high-quality sound and reducing significantly all background noise.