The Keeley Ultra Mod is a professional after-market modification applied to the classic Boss distortion/overdrive platform that greatly expands its tone and performance over the stock unit. Instead of the original pedal’s basic clipping and gain structure, the Ultra Mod reshapes the circuit with higher-quality components and adds a mini toggle switch that gives you two distinct clipping modes: the original “Seeing Eye” style and a fatter, richer Ultra mode. Players report more bass response, smoother mids, increased gain range, lower noise, and a more musical, amp-like distortion that can act like a “Marshall-in-a-box.” The mod also helps tighten up the pedal’s dynamics and makes it more responsive to your playing and guitar’s volume changes.