Unfortunately, upgrading "one tiny step at a time" is not a strategy that is likely to keep you satisfied in the long run. I dare say, everyone here, at some point, has experienced situations when we wanted to upgrade but simply couldn’t afford to spend what it would take to make a real improvement.
It’s not clear to me which aspects of your system’s presentation you’re seeking to improve. If you simply must spend that $400 now, it’s possible a power cord upgrade or vibration control could yield some noticeable benefit, depending upon what your goal is.