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.)?
I would possibly put room treatment / DSP a tied first with speakers. A cheaper pair of speakers could sound much better than speakers 10x the cost if your room is colouring the sound too much. As already mentioned 20db swings aren’t uncommon and there is nothing to remove those other than treating the room or in some cases DSP.
I had a 15db mode in the room which was tamed with Dirac and this made the biggest improvement to my system I’ve heard over upgrading my speakers to 10x the cost of my original pair.