cable connection

There might be no answer to this... but if you had  just one audio cable to upgrade , would it be between pre amp and amp or  pre amp and cd player.
Preamp and source for me. Second choice would be from the wall outlet to the amp. 
Let’s see how many different options we get on this😀
Source <> preamp
Once you lose data upstream, you can't retrieve it downstream. 

Not mentioned, but if the system has a record player, the tonearm internal wiring if it needs it. An uninterrupted run from cartridge clips to RCA plugs.