How to judge a preamp's sound

I just heard a YouTuber review a preamp. He told the audience that he tried it with many amps, and then went on to offer descriptions about "the" character of the preamp (bass, midrange, and treble, etc.).

My question is, Can someone accurately generalize about "the" sound of preamp across a variety of different amps? Wouldn't the amps be enough of a variable to at least complicate the "character" of a preamp? This is a serious reviewer with many subscribers.