A preamp in mathematical terms is a voltage multiplier. It multiplies noise and the intended signal. Resistors generate noise, so do microphone capsules. Mics hear what they hear, and they generate a line level signal that is then multiplied by the preamp. As part of the voltage multiplication, the preamp adds it's own noise based on the design of the particular preamp, components used (transformers, op amps, etc). A cleaner preamp may add less destructive noise as part of the multiplication, resulting in the impression that the preamp allows the mics to hear more of the room, but in reality, it's doing a better job of multiplying what the mics have already picked up. Preamps can't add detail that isn't already there...it's more a matter of how much they degrade the signal as it moves down the chain...some degrade more than others, and sometimes more (or just different) degradation is a good thing.
-mitch