This Thunder Pup from Bigfoot Engineering has 2 preamps allowing swiching between 2 different vibes for the overdrive. Here's what Bigfoot has to say about it:
"The Thunder Pup is not an emulation. It contains two fully fledged analogue guitar preamplifiers, based on two classic British valve (tube) amplifier designs.
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 theVolume control will increase the energy in the low frequencies."