Why do preamps improve sound of CDP?

In 10 words or less... better loading of amp by preamp than by CD player.

You can find a lot of commentary on this if you search through the archives for threads on running CDPs with volume direct to amp, or on threads about passive attenuators.

CDP probably has a poor quality preamp or your headphone amp has a low impedance input (poor quality headphone amp) or you are passively attenuating the signal.

A well designed preamp (high input inmepdance and low output impedance with good output capability) allows you to couple equipment together with no loss of signal quality - this is a well known fact.

Of course some people just insist that passive attenuators or no pre-amps in the path are better becuase of "purity" or "less-is-more" principle and nothing I or anyone can say will persuade them differently.

Depends a lot on the CDP. I found out the hard way (room full of fanatic DIYers) that my own pride-and-joy CDP, a Raysonic CD 128, sounds totally underwhelming through a passive. Sounds terrific in my system, coupled to an Aesthetix Calypso linestage , and I figured it would sound that way everywhere. "Twas not to be. Dave

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