I have to agree with Jim. I really struggle with the concept of having to learn how to hear , and needing long periods of time to begin to hear subtle differences. Yes, some things like fatigue set in over time, but by and large, if the differences are so subtle that they are not "reasonably" readily apparent (a mild concession to time), then you have to question the value of that particular piece of equipment IN YOUR SYSTEM.
Although this will probably create more debate, let me clarify my point on learning to hear. This means, to me, trying to learn to hear beyond the obvious things that will stand out as an immediate change, ie, clarity, detail, accuracy of voice/timbre, pin point imaging, bass depth, overall fullness/leaness of sound. Then there are the more emotional aspects, like being "drawn into the music", or the "musicality" or the "toe tapping" experience. To Jim's point, I was looking for any of these things to jump out at me so I could decide which of these pieces made the music sound better TO ME, albeit with the expectation that the exceptionally well regarded, well reviewed, more costly, dedicated-to-2 channel pieces would clearly show improvement. That they haven't is surprising, and to be honest, disappointing. However, and Tvad and I are square on this now, it is not torture, and it has not discouraged me. I'm getting great sound now, and I'm going to continue to try to improve the sound quality, while enjoying what I have. I may be at a point where I can't hear improvements even if they are subjectively there. It could be my ears, my room, or my system synergy. The exploration will go on to try to determine this. I do believe that everyone is different, and some are just THAT much more sensitive to frequencies, to acoustics, to change, than others. And that sensitivity may have thresholds or plateaus beyond which they are no longer sensitive, or maybe even more sensitive. If you don't get frustrated, cajoled or bullied along the way, the process is actually fun, as long as the bottom line is consistent: THE MUSIC SOUNDS GOOD.