For me, the greatest difference has come from learning to measure my room’s RT60, impulse response, and frequency curves using REW software and a UMIK-1 microphone. Understanding and then addressing these fundamental room acoustics has been transformative.
Compared to these significant factors, specific vibration control for speakers has yielded the most obvious, albeit smaller, audible improvements. However, I’ve noticed almost no discernible difference from electrical adjustments, largely because I’ve never had issues with hum or buzz, nor from other forms of vibration control on components other than speakers.
Indeed, outside of the larger factors mentioned, swapping gear has lead to the greatest amount of new information for me. It is MUCH more of an impact to change or upgrade an amplifier to get a better match with the speakers than to spend much time on electrical tweaks and adjustments.
P.S. I've learned to NOT trust my ears too much. Often, I find myself satisfied with the sound of my system and, if I take the time to measure it and discover (for example) a peak in an important frequency range, correcting that makes the sound even better. So, I guess I trust my ears in the end, but I don't trust them without some skeptical testing and re-listening.