I think that if you start digging into the topic of noise in recordings, you will soon realize that it's quite complex. For example there's really no such thing as "the quietest preamp" as such, since noise levels depend so much on details such as the microphone's output impedance, the gain settings on the preamp, how you "weight" the noise signal as to its frequency spectrum, and whether or not you smooth out the short-term impulse noise the way most manufacturers like to do. Preamp "A" could measure quieter than preamp "B" under a given set of conditions--but then under another, equally plausible set of conditions, the reverse could well be true.
I have seen this myself. At 60 dB gain, one of my preamps is several dB quieter than another one--but at 35 dB gain (far more typical of what I actually use), the second preamp is quieter than the first.
Noise specifications for preamps are nearly always measured with the gain turned all the way up. That usually gives the best numerical result, but doesn't tell you ANYTHING about how the preamp will behave at other gain settings. There is truly no way to estimate, from the noise figure at 60 dB gain, what the noise figure at 30 or 35 dB gain might be. You have to actually measure at the gain levels that you're going to use.
More fundamentally, the noise level of a preamp is essentially irrelevant when other noise sources in the recording "chain" predominate. Many people don't seem to realize that the self-noise of a microphone--even a first-rate, professional-quality studio condenser microphone--will often exceed the self-noise of the preamp channel that it is connected to. The net effect depends on a number of factors, though. For example, the noise from condenser microphones that you can measure at low frequencies is considerable, but is usually completely inaudible unless the signals are amplified greatly, because our ears and brains just don't register low-level, low-frequency sound with anything near the same sensitivity as we register midrange sound.
So these are just some thoughts for starters.