this might be a helpful test for minimizing the variables when comparing preamps or mics: put the mic up to your monitor and record an identical excerpt of a complete full range mix for each item in the test.
I had read that this test is not helpful for comparing converters though, as it has already gone through a generation of A to D. Perhaps the same could be said for mics and preamps, but so far I think it can be helpful to some degree.
for converters, fortunately I have a preamp that has up to 3 outputs (speck micpre) which I can send to different converters and then compare the identical live take.
I remember reading somewhere that someone had success with a kind of mic Y-cable that could then be sent to two different pres. I don't recall if it was suggested to split right after the mic or just before the preamp. (hmm, maybe I read that here, searching...)
...ok, well, on a quick scan of posts, the y-cable thing seems to not have unanimous support as an approach. Buying a mic splitter box could be a more reliable option, but these may not be just a cheap buy, such as this one:
Radial Engineering - JS-3 Jensen transformer equipped mic splitter