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.)?
You need to put some reasonable parameters on this. If you *remove* any part of the system it will not work. If you get the worst possible option for any part of the system it will be bad. Speaker wires don’t matter unless they are 200ohm ultra-high inductance wires running next to your mains cables - then they are the worst part of the system.
However if you take some kind of typical Hi-Fi system then it is extremely likely that the speakers are the worst component in terms of linear and non-linear distortion. It is pretty easy to make very good amplifiers, sources etc