I fully agree with Swampwalker: once you select the right speakers for you and your room, then the speaker/amp interaction is the next thing to get right. Matching the load of the speaker to the drive capability of the amp is key, and then you also need an amp that matches the sonic palette the way you like.
And, yes, I also think the preamp is "the heart of the system". And who can argue that if the source cannot deliver it, nothing downstream can make up for that, so....
Oh, and I have allocated ~25% on the cables in my system; certainly not by plan, but by happenstance from comparing various options in my system and finding that they often make as much of a difference as a solid component upgrade. I would never have predicted to end up in that circumstance, but I am happy that I did, given the results.
In the end, everything matters - - but I do think starting with the speakers in the room makes the best sense (certainly has for me).