Why do people have a need to frame the importance of individual pieces of audio equipment as a mutually exclusive argument? The fact is that in audio every piece is important, and to claim that getting one right is most important is useless. If your source sounds like shit, the sound you hear will be shit no matter what else you've got. If your speakers sound like shit, you'll hear -- you guessed it -- shit, no matter what you're using as a source, amp, etc. Who gives a crap what flavor the shit is?
One of the questions posed here was whether to upgrade the amp or speakers first. That's a question of strategy, not importance. I think it's easier to start with the speakers and then find an amp that is capable of driving them and producing the sound one is looking for. Others may prefer a different strategy and that's fine if it works for them. At the end of the day it doesn't change the fact that every link in the audio chain is vitally important. I would hope we at least can all agree on that.