Why do preamps improve sound of CDP?

I have tried listening to my CDP through my preamp and bypassing my preamp going from CDP directly to headphone amp. If I go through the tape out of my preamp the sound is even nicer than direct from CDP to headphone amp. Why is this?
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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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I agree with Shadorne in principal that a pre-amp SHOULD be better than a passive, as Chris Bryan put it in the July-Sept issue of HIFICRITIC 'The very best pre-amps are definitely superior to the even the best passives, volume potentiometers or switched resistors." The problem is the cost of "the very best pre-amps". Some quite expensive ones have failed to equal a passive; in the test following the previous quote Bryan found that the Aesthetix Calypso,Bryston BP26 and Roksan Platinum PR15B all failed to equal a Audio Synthesis Passion passive, which scored 30 on their scale. The Aesthetix did best, 20 with standard tubes and 26 with selected older ones. The Bryston scored 16 despite measuring very well and the Roksan 20. The very best pre-amps , such as the C-J CT5 score 90 or above on this scale. The Creek $500 passive scored in the high 30s. So you can pay out several thousand dollars and not equal the sound of a good passive but the best actives are considerably better. This is in line with my own experience, I am currently using a passive myself. BUT I have amps that work well with passives [not all do] and while I have good pre-amps on hand they are not in the Super class. So again as in so many things in audio there are no simple answers, passives ,in the right system ,give a lot of bang for the buck, best actives at top. Even there exceptions exist, in the same issue they found that the dCS Scarlatti CD player sounded better direct than through the AR Reference 3 pre, which is the best pre they have tested. DISCLAIMER, for all I know YOUR system may be entirely different but I think the proceeding may shed some light on why there is so divergent a range of opinions on this subject. It is my own feeling that pre-amp design has not advanced as fast as amp design and there are some very overpriced ones out there. I AM not talking about YOURS so put that brick back in your pocket. I have omitted all mention of the variation in the quality of player output stages, it is a whole topic in itself.
Looking at my post I should have emphasized how much this is a matter of taste and associated equipment. Before I purchased a Transparent Reference digital cable I was using a Musical Fidelity NuVista to drive my Spendor S 100s. The passive system [ Audio Synthesis Passion Ultimate, Meridian 605s] sounded a little underwhelming, just as Dave experienced. Now the MF sounds a little too forward and "in your face" while the passive sounds laid back but VERY revealing, much easier to listen to. BUT, I suspect many would still like the MF better, as I well may the next time I change associated equipment. A good system is like a kaleidoscope, every change you make gives a new pattern.
More gains, better loading. But if you think a preamp is not your thing then a audio buffer also does the job with a little less gain. Some said using an audio buffer instead of a preamp actually sounds better. more transparent etc.

I received my preamp today and I can answer my own question. The preamp improves the sound of my headphone amp. It dramatically improves the sound of both my tuner or CDP. I had two headphone amps going attached directly to tuner and cd player and I've been listening for a week waiting the return of my preamp. On it's return today I notice both the tuner and the CDP do sound a lot better through the pre even just going through the tape out. The improvement is large with the pre adding character to the music making it sound good basically. I can't explain it but the headophone amps alone were not that good sounding until the preamp was put in between both tuner and cdp. Now both sources sound like they should. The difference is not subtle.