Could someone explain quality sound in preamps?

As I understand it a high quality preamp should let a signal pass through it without adding any of its own "sonic
signature" or coloration. So why tubes or solid state? Why not just passive preamps? In other words can a high quality preamp add a coloration that would be considered an improvement in sound quality? I'm thinking of getting a high quality preamp but a little confused. I would like to smooth the sound ever so slightly. Anyone? Thanks.