Shadow_7, if you eliminate the preamps with audibly non-flat frequency response, audible levels of distortion within the dynamic range of interest, or that have incorrect or inadequate phantom powering and/or too low an input impedance, and if you make really careful comparisons using IDENTICAL input signals (something I'll get back to in a moment), the sonic differences among preamps tend to become so small that one can reasonably doubt whether they exist or not.
You could try to compare two preamps by placing two very well-matched microphones side by side and recording the same occurrence--music, speaking voice, gunshots, jangling keys, or whatever else you like--with each microphone using one of the preamps. Take the two output signals, route them into the same two-channel A/D converter, make a two-channel recording and then (using proper dither) normalize the two signals so that they have identical levels. Then you could compare the two recordings; for that matter you could subtract one channel from the other to hear and see and analyze the response differences.
But in large part you would very likely still be seeing/hearing/analyzing the response differences between the two microphones and/or the specific positions they were in (even if they were less than 2" apart), since those differences are often an order of magnitude greater than the differences between any two high-quality mike preamps. The better any two preamps are, the more likely they are to sound the same, if you define "better" the way I and a lot of other people do (although clearly, not everyone who calls himself an engineer sees it that way).
"Mike pre shootouts" where a singer sings or a guitarist plays in front of a larger group of microphones, each feeding one channel of one preamp, are certainly a total loss; if you hear differences, you cannot tell to what extent they really reflect on the preamp, the microphone, or the microphone's position within the group. Worse yet are the ones in which someone repeats the same music or speech--no human performer is consistent enough for that to work, not even Rachael Flatt (who I still think should be accompanied by Earl Scruggs).