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.)?
Speakers and those things called ears. Protect your hearing at work and play. Every concert that results in ringing in your ears causes permanent damage.
For better or worse I have found it harder and harder to discriminate between excellent and more excellent equipment as I have aged. I can still appreciate subtle differences at the bottom end, but not the high end. For worse because i don’t get the same spine tingling chills i used to get from very high end systems, for better because I know longer throw good money after bad to achieve better and better sound. I just had a formal hearing test and my hearing is on the better side of what a typical 63 year old can hear, but everything above 12kHz is very depressed, confirming my subjective sense of what I no longer appreciate in great music systems.
TLDR - take care to protect your own cranial transducers….and good equipment will sound even better