The Visual Sound Truetone preamp uses a charge pump to raise the supply voltage from 9V to around 25-26V. When my buddy RG Keen (who designed it) wrote to me to describe it shortly before it was released, he said that "amps tremble in fear" when it comes near.
Even though it wasn't designed to produce any clipping on its own, that relatively unrestrained headroom provided peaks that few amps could content with without
producing distortion themselves.
Incidentally, as many designers have found over the years, it is good to have some form of treble-cut in a preamp. If amp distortion adds
harmonics of what it receives, then you don't want it to be adding harmonics of harmonics (which few of us like). So having a treble-cut on the boosted signal removes that high end you don't want the amp to know about, and makes for a more desirable amp overdrive tone. In speaking with Bill Finnegan at length about the Klon, he told me he intended the Treble control on it to be used for rolling off highs, in order to produce a more desirable amp overdrive.