I own one, and it's the most metallish preamp-distortion pedal I've heard. Some amps, such as some Mesa-Boogie amps, enable you to instantly switch between power tube types, such as by using a footswitch. I bought one. I have a lot of experience with distortion pedals, and this one is definitive. It's important to understand that overdrive pedals are used to drive a tube amp's input stage into distortion.
Distortion pedals generate distortion themselves.
Fuzz boxes boost the bass before distortion, for a rough, bass-dominant breakup.