:-)
I'll offer a differing opinion.
I think the term "warm" is often misapplied and perhaps reflects some hopeful thinking because the sound of a modern "clean" (and I don't actually agree they sound clean) generic IC application data sheet circuit leaves some people yearning for something that excites passion.
I've got a variety of preamps that were designed and built by guys that are passionate about the sound of preamps.
They each sound really clean yet they all have a little something-something that they do. It's not like they provide some sort of static EQ curve or explicit distortion effect, which is what I think a lot of people assume you'll experience. What they offer is a range of tonality that is linked to impedance and reactivity. You experience a dynamic frequency response. It's not dramatic, it's subtle. If you get used to having something that reacts just the way you want it then you miss it when you use something that doesn't.
None of my guests are tech savvy, gear savvy, or brand name aware, but they each respond to different mic and preamp combos and when I find a combination that they enjoy they get a great big smile on their face. I try to sense what they are pushing or searching for in terms of tonality while they perform and I try to give it to them up front so they can be inspired by it as they do their thing.
I like the fact that none of my guests have abstracted preconceptions about the gear I provide for them because it reaffirms my thoughts about the idea that the gear does indeed have sound characteristics that can be described. When someone smiles and says something like
"wow, I like what that is doing to the low notes" or stuff like that I get a big smile on
my face.
IM
HO, if you make a practice of bringing stuff in with the intention of exploring options later you are undermining the performers ability to exhibit their most personal expression and you are elevating the role of "mix engineering" in a way that may usurp the ability of a performance to seem compelling on its very own merit.
In other words, I think having a dedicated preamp and getting to know all about it is a great idea. :-)
all the best,
mike