I don't know what preamps add beyond gain, but they do (perhaps it's their on character). It's like power cords. How the hell could they make a difference; it's just electricity. But they do! I've run a CD player with volume control direct and with a preamp. I liked attributes about both, but preferred using the preamp. The direct route was cleaner, but using the preamp made it more dynamic. I've experienced this when comparing active and passive preamps. Mahandave is correct; expensive preamps are worth it. They pass on the siganl "as-is" from the source while doing the preamp thing. I should add that I prefer tube preamps. They provide a realistic soundstage. Also, preamps allow using multiple sources (FM isn't dead yet!).
The Truth About Preamps
I'm wondering now that we're in the digital age with so many cd players having variable outs, what does a high quality preamp do for a system? I've heard so many different stories on either you get more detail straight in or that you get more bloom with a preamp. I would think that if you had a musical cd player such as a Cary running straight into a musical and smooth amp such as a Classe or Conrad-johnson you wouldn't need a preamp...all this given you have only cd as your source...I'm just wondering what you guys think and your experience running a cd player straight into an amp. I'm thinking of running a Theta Miles straight into a Classe CA-300