What made the biggest, most noticeable improvement

The biggest, most noticeable improvement? Well, lessee...

First it was the speakers
Then it was the amp
Then it was the cables
Then it was the room
Then it was the treatments
Then it was the cdp
Then it was the DAC
Then it was the preamp
Then it was the amp again
Then it was the speakers again
Then it was the DAC again...

Get the point? The point being that any component or adjustmet to the listening event carries the capacity to be perceptually the most impressive improvement encountered up to that time. I'm not saying they all do all the time; one must select well to get such a gain in a component.

IOW, it is a fruitless endeavor to seek a magical component which supposedly will confer the best gain or improvement. I have heard representatives from all of the above with the capacity to stunn in terms of improvement of a rig's sound.

Aaron, perhaps we are speaking past each other; I was referring to genres of components, not any one system I have built. I have built dozens of systems with a high turnover of gear, so I have indeed experienced all of the components I mentioned as perceptually conferring the most impressive change at the time. Inevitably it gets usurped by another as the rigs change.

If tweaking our semantic understanding does not close the gap in our conclusions, then it's likely we will not see eye to eye on this, which is ok. :)

One of the points I was hinting at is that when it comes to perceived improvement we are creatures of the moment and that what strikes us as impressive in that moment easily takes predominance in our minds. It is difficult to state with certainty that something we were impressed by months ago was less impressive than what we were just impressed by. :)

IMO, it's playing in quicksand to discuss perceptions of efficacy of one genre of component type (i.e. DAC vs. Preamp vs. Speaker vs. cables) over another, for it's difficult to extract the one and assess it apart from the system! ;) In the development of a single rig, sure, that may be seen, but extension of the experience to all audio systems is hard to defend.

For that reason I consider it a fairly useless endeavor to try to gain consensus in regards to a component which is able to confer the biggest advantage and seek it, much less apportion the budget accordingly (Maybe it would be acceptable to do so with speakers, but little else). Get the right amp and a person is likely to say the amp can make the biggest difference. Get the right speakers and an individual may conclude the speakers are most important, etc.

As usual, YMMV and I'm not interested in arguing my experiences in it further. :)