preamp vs amp: relative effect on sound?

Recently, there was an interesting thread on combining a tube preamp with a SS amp, which is what apparently a lot of people do.

So two questions.

Q1: in a preamp + amp combo, does each unit affect the eventual sound equally, 50/50, or is one going to affect the sound more than the other?

Q2: in a combo of higher gain preamp + lower gain amp, will more gain being provided by the preamp have any effect on your answer to Q1?