Sonic signatures are like written ones, very individual, and very detailed. You can talk about them in generalized terms, but those descriptions won't get you the real picture. In a written signature you can talk about upward or downward slants, loops, curves, etc., but that won't help anyone visualize the actual signature.
People who talk about sonic signatures in terms of V or U response curves are doing exactly the same thing, either from over simplification or lack of understanding. The response curve is the real key. To really get the idea of what a signature sounds like from a measured response curve you have to make the connection between indicated frequencies on the graph and what they sound like, then understand the interaction of the width of a peak or dip with it's magnitude. Basically that relationship is smaller width peaks and dips are less audible than wide ones, and so the narrower the excursion the deeper or higher it can be before it becomes audibly objectionable. How audible a response variance is affected by where it is in the spectrum also. At the extremes, variations are harder to hear. In the middle, fairly easy.
The hard part is trying to imagine what a particular curve sounds like. But for that, we have new tools on our computers, like editors with equalizers that give the user not only a graph of what it's doing but also the ability to modify the width (Q) and degree of boost/cut. This can help you to audition the sound of those types of response changes, and little by little make sense of the measured curves indicate something about the signature.
Of course, frequency response isn't the whole story, but it's the biggest part of it. If you can understand that, you'll get most of what the signature sounds like.