The Scare Crow is a compactness double gain stage overdrive. It was designed to have a wide variety of sounds in one effect and is perfect for rock and blues solos.
By mixing pre-gain and gain you can get the best frequency response for your guitar.
Using only the Gain stage gives a more vintage sound suitable for blues and rock rhythms.
The Pre-Gain stage, on the other hand, gives the sound a natural saturation that makes it more open and bright, suitable for modern rhythms and arpeggios.
The active tone allows fine tuning of the high frequencies for total control over the sound.