I would add the following:
Imagine your amp to have a certain type of distortion. Perhaps it adds a graininess to the sound.
If you send it a signal that is also grainy-sounding, then the amp's graininess will not ADD to the graininess in the signal, but will MULTIPLY it. You are thus hearing DISTORTED DISTORTION, which is really bad.
This is why we recommend your best cables be placed at the source. There are far too many cables out there having a 'grungy' (dirty) sound that most of us would attribute to poor digital reproduction.
By using a cleaner/better cable at the beginning, everything downstream breathes more easily, since all are receiving less distortion to then re-distort.
I think that avoiding 'distortion multiplication' is part of the synergy reported for certain combinations of gear, speakers, and cables.
Best,
Roy