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?
no_slouch
A good pre-amp doesn't improve the sound quality, it just does not degrade it as much as a lesser quality pre-amp.
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I have tried going direct several times but having the preamp in the chain has always been better. Arthur