My current system is the best I've ever had. It's very well balanced, meaning that none of the components are clearly the weak link, and none stand out by revealing the rest of the system as being the weak link. I'm enjoying this state of affairs for the time being, and buying a lot of music with the funds I might have recently put into new gear.
If I do ever change (as opposed to upgrade), it will be for one of two reasons. The first reason is that the amp I have runs hot, and it heats the room. I'd love to get the same quality of sound (on the music I listen to) without all the heat generation. This would almost necessarily imply new speakers, and then the ball is rolling. Second, I'd love to play with some different pairs of speakers - I may end up deciding what I have is what I like best, but it would be fun to buy a pair or two in addition to what I have, experiment, sell the ones I don't care for as much, buy some others to experiment with, etc. This process could, again, get the ball rolling. -Kirk