If you are not happy with the sound, there are two things that may help. First, make sure you are doing the best you can with the positioning of the speakers and with rugs and so forth. In other words, deal with the acoustics of your space as best you can. Other than that, replacing the speakers would be the next thing to do. But the best speakers ever made will not sound great in a bad listening environment, so make sure you do not have any nasty echos or other such problems with your space before you buy new speakers. I would strongly recommend not bothering with a new preamp, CD player, or power amp (unless the speakers you end up with require something more). Unless one of them is actually malfunctioning, any "improvements" you could get from replacing them is likely to be imaginary.