Ok this will be a good thread.

From a logical viewpoint, there's no doubting the importance of the source, as Twl notes. In absolute terms, garbage in, more garbage out.
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From a PRACTICAL, "systemic" point of view, however, things *could* vary.

Practical as in, "which is the weakest performer in the(my) system?" OR "...in ANY system?" whereby "imporving THAT component will invariably make the MOST PERCEIVED difference"???
I think that MOST 'philes respond to the practical, everyday life, question rather than the absolute...

Possibly because information retrieval, such as it is, is at reasonable level in most of our systems. Note in this respect that in digital, most comments about a player talk about "nuances", musicality, etc, rarely about information per se.

My take is that in many cases our sources are equal if not superior to the task. In order to put these to their best use, we need outstanding components down the line, too.

Are we losing info, or "sonic qualities", along the way to our ears? I think so. If so, a pre-amp and a speaker *could* be doing the most damage...

In this respect, taking care of the pre *could* bring the system to another level, perhpas even surpassing the source, driving the speakers to their limits of resolution & musical qualities... Hence then, in THAT system, the MOST important "component" becomes the source (again) or the speaker, etc.

Bob- Twl's reference to digital IMO serves as an example only -- I used it in my post for, probably, the same reasons.

IMO, the goal we all have is to achieve balance between components' capabilities in a way that no component reveals annoying weaknesses in another...

In this respect, the changes that analogue upgrades/setting up bring about usually are (IME) more immediately perceptible downstream -- hence the use of this example. To the point where changes are subjectively perceived as being more & more subtle -- let's say that we have made improvements to the point where we reached the limits of the downstream system...

Ultimately, however, speakers are our "primary communication source" with the system, as you correctly point out. So, practically, we can only improve the system to the limits of what these speakers tell us (or don't)-- or, improve upon the speakers... Which MIGHT bring us back to the drawing board, looking for amps to drive said new and better speakers. Etc.

OTOH, I agree that many speakers manage mid-range reasonably well (Twl) and would add that, most speakers err considerably (as you note) when it comes to reproducing info on both extremes -- especially the upper highs. Sigh.

At this point, I've come to believe that the "most important part" is clearly system dependant. However, if I were to set up a system FROM SCRATCH, I would now look at the two ends of the chain first...