Thank you for your responses! I still think that the solder acts as a "bottleneck" as current flows throught the cable, i.e. the copper cables have a conductivity of a 100 and the solder of the order of 40 depending on its composition. In other words the solder has a much greater resistence to the current flow than the copper. Am I mistaken?? Therefore I think that by having a expensive/exotic cable is a waste of money since ultimately the solder resistance is the one that determines the "quality" of the current transfer to the device being feed. Any more comments?
Yes, you are mistaken. Conductivity, and its reciprocal resistivity, do not cause bottlenecks, and should not be thought of that way. That is a common misconception.
The magnitude and the effects of resistance are proportional to length (as well as being proportional to the resistivity of the material, and inversely proportional to cross-sectional area). And the effects of resistance are also highly dependent on the impedances and other technical characteristics of the circuitry at both ends of the connection.
For example, even absurdly large amounts of connection resistance (for example, several ohms) are unlikely to have any significance in the case of a line-level interconnect cable, because they would still be totally insignificant in relation to the input impedance of the destination component (and probably also small in relation to the output impedance of the component providing the signal). That would mean that the fraction of the original signal that is lost across the resistance would be completely negligible. And, hypothetically speaking, even if it weren't completely negligible, as long as the input impedance of the destination component does not vary in a major way as a function of frequency, the loss would be compensated for with a slight tweak of the volume control.
Which is not to say that a poor connection cannot degrade the signal in other more subtle and less predictable ways. So the comments by Tim and Rodman represent good practice.