There is no way to "clean up" an analog signal by running it through a few stages of amplification. There were (are?) active devices like Carver's Phase Linear Autocorrelator which was a dynamic noise filter which "cleaned up" LP rumble and surface noise quite effectively, but I don't think that such circuitry is included in a preamp (other than the Phase Linear preamp).
In the digital domain, cleanup is possible to correct various digital parameters like timing.
However, a preamp can change the sound, and perhaps in a way that sounds good. Usually this is the result of harmonic distortion, which adds harmonics similar to those produced by musical instruments. For example, some violins produce a different harmonic signature than others when played in exactly the same way. The harmonics that are missing in the inferior instrument might be added in the preamp. However, IMHO, this sort of sound improvement should have been done by the recording engineer when the media was created.
In the digital domain, cleanup is possible to correct various digital parameters like timing.
However, a preamp can change the sound, and perhaps in a way that sounds good. Usually this is the result of harmonic distortion, which adds harmonics similar to those produced by musical instruments. For example, some violins produce a different harmonic signature than others when played in exactly the same way. The harmonics that are missing in the inferior instrument might be added in the preamp. However, IMHO, this sort of sound improvement should have been done by the recording engineer when the media was created.