A good preamp can improve the impedance matching between sourced and amp, and also give the music more body and life, primarily from extra gain. I've tried it both ways and have preferred a good preamp every time.
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?