The DOD Overdrive Preamp 250 is a 70s-designed overdrive pedal, with a gain knob turning up the dirt and a level knob setting the volume of the output signal.
With the level set high and gain at minimum, you can use the pedal as a clean-ish booster with a semblance of grit to the sound.
That grit turns into more amp-like overdrive, with not much in the way of tonal shift as you turn up the gain, delivering a wide range of possibilities depending on how you combine the two controls. Basically, it adds crunch to a clean amp or drives a driven amp further - which it does very well.
There's more, though: turning the gain knob past vertical and on to its final reaches sees the distortion get brighter and more aggressive as the nature of the diode- clipping circuit starts to offer some of the character you'd get from a fuzz pedal.
There's a schizophrenic feel to this pedal, but that's not a bad thing: just set the gain for the sound you want.