Having just upgraded my amps and then my preamp, I found them to be roughly equally important, but each effected the system in profoundly different ways. For me it came down to what the amps do and what the preamp does not do. My new amps control my speakers much better than the old amps did. Bass is tighter, deeper and more focused. Dynamic range increased as well as improved soundstage size and definition. My new preamp gets out of the way much better than my old pre did. It is quieter and more neutral. Micro and macro details improved, timbre and transients improved. However, each upgrade made the listening experience more involving in roughly equal amounts. They are from the same manufacturer, so they work very well together. My system is built around my electronics. Its the only area I have really upgraded over the past six years.
Having said that, I have found the most critical component/relationship is the room-speaker interface. This is about scale, placement and taste. Then the speaker-amp/cable. And finally the amp-preamp. The front end is a completely different thing and can be thought of as its own system(especially analog). It can be of equal or better quality but is not as interdependent as those other components are and can be thought of independently from the rest of the system.
At least, this has been my experience having just upgraded my electronics.
Having said that, I have found the most critical component/relationship is the room-speaker interface. This is about scale, placement and taste. Then the speaker-amp/cable. And finally the amp-preamp. The front end is a completely different thing and can be thought of as its own system(especially analog). It can be of equal or better quality but is not as interdependent as those other components are and can be thought of independently from the rest of the system.
At least, this has been my experience having just upgraded my electronics.