Gonna see if I can ask this question correctly:

A phono cartridge can change how a vinyl record sounds quite significantly. But if I have to choose between upgrading to a high end cartridge or to a high end stereo, which will make a more noticeable difference and how?

Now with CD’s, what would greatly alter the sound the most? The player or the stereo?

The speakers are also at play in this and even wires and all the pieces like pre-amps, etc. when I’m asking this question.

But what singular piece of equipment can effect how something sounds, and will it change from source to source (cd’s, vinyl, digital, etc.)?