Bigfoot Thunder Pup pedál 

The toggle switch offers two distinct flavours: The Big Amp mode has a natural low frequency compression and power supply 'sag', combined with a muscular, forward sounding lower midrange push, typical of a 4x12 stack. The Little Amp mode has a faster attack in the low frequencies, with more air in the highs, and a spring to the mids that stays cleaner for longer, typical of a 2x12 combo.
The Thunder Pup's dials react in the same way as the preamp volume and master volume controls on a classic British valve (tube) amplifier. The Gain control will increase the presence and bite by exciting the high frequencies, while the Volume control will increase the energy in the low frequencies.