Yes pre-amps amplify the signal. Not all Pres are made equal with some significantly better than others in sound quality. There are Pres that also pleasantly add colouration to the signal, this can be a good thing for one signal and may not be as desirable for another. What might work for one vocalist with a certain microphone with a certain style to suit a song might not work for everyone or even for every song. But what you've been hearing about is no doubt regarding expensive pre's made by companies like Neve, SSL, Focusrite and Avalon ect ect ect.... These will add this favourable colouration to certain signals in the right scenario.