A system by definition, requires careful and often numerous component change-outs to offer the best of the individual parts. That being said, I find the speaker the most visible of flaws. Maybe, that's due to it's revealings of up-stream anomolies or component mis-match, but speaker substitution often brings an entirely new presentation to the system. Then again, without an excellent front-end, following components falter.
But the most critical piece in a stereo system has to be the music.
But the most critical piece in a stereo system has to be the music.