which component has the most potential to improve a stereo system?
I think it depends on system deficiencies.
I have B&W 804`s teamed up with an Acurus A-150 power amp. (a BIG no-no!).
In order to rectify the harshness in the upper frequincies, I inserted a tubed preamp into the system. I `ve taken a sows ear and made it into a silk purse.