Re: Clean Boost?
My understanding is that the pre-amp stage in a valve amp, is often a multi-stage affair. Even with a single tube, the signal is gained up, then fed in again (to the same tube, but to another separate "side" of it) the resulting amplified signal is then sent onward to the power stage.
Your guitar signal arrives and is whispering then talkin then shouting. Certain character is added as we go along. If you clean boost first then the amp gets a guitar signal that's already talking. Different character. And the overdrive sound will obviously happen sooner as it breaks up.