Mceljo, you tried to be too analytical of the situation and hence used maths to evaluate the situation.
On the contrary audio is not just about science and maths, a large part of it is ART. There are many aspects of audio which is intangible and cannot be calculated, measured or justified by science because music touches your senses directly and how it is evaluated after that is totally ART.
I would answer your post in two parts, 1st will be scientific and 2nd would be non-scientific.
1. To answer your question using basic maths and science, the signal that travels between the CDP and the preamp is extremely low level, a slight contamination to this signal is actually big change in terms of % of change. Moreover it will go through two levels of amplification (preamp and power amp) so the change is also amplified and in turn magnified to make a huge impact to the final sound. It is a big challenge for the interconnect to carry such valuable low level signal maintaining all its integrity.
However the signal from preamp to power amp is already amplified and is a high level signal (relatively). The effect of contamination is much lower because the % change compared to the full signal is lower and also there is one less level of magnification i.e the power amp. So the overall impact is much much lower. Hence the compromise at this level will not be as drastic as the interconnect at the first level (cdp to preamp).
2. Coming to the non-scientific explanation. So, you said "assuming that the CDP gives out a perfect signal", my first question would be, how do you know what a perfect signal sounds like ? For example, one interconnect may sound extremely holographic with precise imaging of instruments with nice detailing and tones While another interconnect may bring in amazing body to the instruments, great tones, warm, open airy stage but little less detail and little less precise imaging. Tell me which one is right ? Meaning which is the sound that mimics the CDP's perfect signal more ? Very difficult to say right ? Basically we do not know.
All we know, is the signature sound that appeals to our senses more. When you hear that signature you call it MAGIC!! That is the magic I was talking about in my previous post. Even our equipments are bought with that very same magic in mind. Unfortunately cables play a big role in deciding the final sound (considering you have a revealing enough system). Hence one needs to extend his search for that magic even with cables. Now, the effect of the first interconnect in the chain is the most drastic. It decides the signature of the system by a large extent. So, if you know of a cable which brings in that magic to your system, buy it if you can. The second cable may reduce/change the effect by an extent but you still get a good part of that signature sound. Sometimes it so happens that it is a combination of two cables which brings that magic in but that is a different situation.
So, it is not really just detail, dynamics or staging, it is a complete presentation that one looks for and that if one cable is able to lock-in in your system, get it.
