OK, now we are getting somewhere!
It looks like you have some nice gear there, and have tried a fair number of different cables/ICs in your system, and in at least one instance heard a difference. but usually not.
In my case, I am able to hear slight differences with ICs and speaker cables, prefering all solid core (magnet wire and CAT5 DIY) mixed with some Kimber Heros, the most expensive IC I have at ~$200/meter.
The differences I hear are subtle, but present. Sounds similar to your case. So why are we arguing?
Edit: To be fair, since you answered mine, I'll answer yours:
CAN a difference in cables exist?
Yes
SHOULD a difference in cables exist?
in an ideal world, no. They'd all be perfect.
If differences exist, WHY do they exist?
Contruction methods and materials would be my best guess.
CAN differences be measured?
Some can.
CAN we hear things that cannot be measured?
I don't know.
CAN things be measured that we cannot hear or sense?
Yes. UV light, sounds above 20kHz, etc
CAN two things that are measured to be identical be proven to be perceived as different?
No. How do you prove perception? But if six audiophiles all say just one of the 'identical' things sucks, then it sucks. And you've measured the wrong thing.
Can perception be verified with scientific testing (NOT belief; actual perception)?
No
BINGO!
By looking at our answers to those questions, you ask why we would argue with each other about cables.
We really shouldn't be, as our beliefs are pretty much the same (cables CAN sound different, but IDEALLY they shouldn't).
I'd bet a lot of pople would find it to be the case if they posed the questions to each other (or would fiund they are arguing about two very different things.--- "My Mitsubishi handles better than a Ferrari" "Yeah, well the Porsche has more BHP than the Mitsubishi".).
They say the best way to defeat an enemy is to KNOW the enemy.
We're not all enemies here by any means, I just mean it makes sense for me to know a person's rationale for their answers and decisions when I discuss/debate with them.
I WILL state I disagree with you on the fact that perception CAN be proven with scientific testing only because it's been proven that it can, even when the test is done with just one person.
For perception (I guess perception of CHANGE would be more accurate to say)to be real, you can prove that you perceive a change by being able to recognize that change with a higher than 50% success rate when compared to a "base measurement".
If you can't do it, then you are not really perceiving a change (i.e. you can state that you perceive a higher chance of flipping heads versus tails when you wear a blue shirt, but if you flip the coin 100 times in blue and 100 times in red, you'll find that the odds still are very close to 50%. You can state that you perceive a change when you switch from blue coated magnet wire to red coated magnet wire, but unless you hit a better than 50% success rate of knowing whether the switch was made or not without being told, you haven't really perceived a difference).
When talking IC's, if two measure identically (Voltage, Current, Resistance,
Capacitance and Inductance, and anything I left out), they SHOULD "sound" identical.
If you hear a difference, you should be able to identify that difference when it occurs with a success rate higher than 50%.
If you can't, then you've shown you can't really identify the difference.
If you CAN and have verifed it, then it must be that the signal is in some way altered by the cable (or it's interaction with the components, which CAN be proven).