How to judge a preamp's sound

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?

Yeah, I’d agree with that. While it’s certainly interesting to hear how a pre might pair with a given amp, if you’re interested in the relative sound of a preamp the logical, direct, and more common reviewer methodology would be to compare it to other preamps. Duh.

If you’re trying to decide on a sports car, do you drive one car on a bunch of different roads to decide which one you like or would you compare several cars on the same roads? Sure, in the first case you’ll get a good idea of what that one car can do, but it’s not as helpful in providing a relative perspective and/or making a buying decision. And after all, isn’t that the most important information a review can provide to a reader? Reviewing one preamp with a bunch of amps is interesting and somewhat helpful, but IMHO it’s less effective than comparing one or more preamps to each other. Humans are inherently poor at judging things on an absolute basis and much better at judging relative performance. Every reviewer has had the experience of thinking they have the sound of a component nailed, then they substitute in a competitive product and they realize how off they actually were in their initial impressions. Without a comparison they’d have been way off in their assessment. This is why I always take reviews written without a comparison example with a HUGE grain of salt. Anyway, just my take on it.