Getting the book Robert Harley's "The Complete guide to High End Audio" would give you a good introduction to the basics of electronics and many other useful bits of information. For me pre's have always improved the sound. Without pre might be a bit cleaner but with is richer.
why does preamp improve sound quality?
I recently listened to a Mark Levinson no.390s CD processor directly connected to a pair of Quad II-forty five tube amps. When a matching Quad preamp (much cheaper than the Levinson) was placed between the Levinson and the amplifier, the sound improved dramatically even at moderate volume- it became much more clear and transparent. Why would this be the case? Wouldn't adding an extra piece of equipment add more distortion?