I suggest you need to establish your objectives. Once you get above the kind of gear you can buy near the checkout line at Best Buy (/s), it's not really about general improvement...rather it is about a direction you feel you need to push your system to.
What is it you are looking to do? More bass? Sharper transients? Image/soundstage? Wider sweet spot?
Much of this can be accomplished by looking at the room and setup and not about different gear. Toe-in, toe-out, move out of corner, what have you.
That said, I do agree that speakers make the biggest difference if you must swap out gear.