How are you hearing no difference?


In my experience, I've never heard two pre-amps that sound exactly the same, nor two DACs that sound the same, nor two amps...etc. Yet, occasionally someone will claim that they heard no difference between Product A and Product B in their system.  I find it difficult to believe.
128x1284hannons
I need a psychologist to weigh in here. (I am not one). The central fallacy as I see it is; assuming one can hear a difference in presentation between two similar components, is one better? Chocolate and vanilla are both ice creams, both cold and sweet, but decidedly different; so which is "better"? Can't they just be different? I prefer vanilla, so to ME it is "better."
This is the tyrrany of choice; we assume that because two things are demonstrably different one has to be better. Take tubes and transistors. They both distort, but in different ways. This is what gives them their characteristic "sounds." But as they improve in quality their distortions minimize and their sounds become more similar. Still, each has a distinctive "sound" of its own; perhaps a vanishingly small difference but since there are two different pieces being compared then at least a theoretical difference. Which is better? The one I prefer may not be what you prefer, but for me the better component is the one I prefer, and I don't really care what YOU like best. 
I find audiophiles divide into two groups; those that listen to music with components vs. those that listen to components with music. 
The problem is too many people make this way harder than it has to be. A lot of people listen for a lot of different things.  For me it’s simple - I listen for clarity/resolution/detail - basically, do I hear things I’ve never picked up or heard before and is it more intelligible?   So to dohanian’s point, chocolate vs vanilla = Speakers are the instrument - do you want a Taylor, Martin, Gibson etc. guitar?  Find the speaker that tonally appeals to you.  Now, who do you want playing your instrument - your Electronics?  If you liked the Taylor guitar and handed that same guitar to Jeff Beck, Eric Clapton, and Chet Atkins, it’s going to sound very different, but for one of those guys, that guitar is just going sound right with them playing it.  So tubes, or solid state?  Yes!  One will be more right and “Does it get out of the way” - or does it add or take away from what you’ve heard in that recording you’ve listened to a 1000 times?  There in lies your answer - yes it’s better or no it’s not - because it certainly will without a doubt be different one way or another, however, different doesn’t always mean better or worse, but like I said, there is a synergy that can happen with Electronics and Speakers that can just be great and then you should know.  That’s also why it’s important to pick well recorded music to audition with because then understanding flaws in recordings of music you truly like to listen to becomes easier to discern.  The Music has been mixed, mastered. EQ’d, engineered and produced, so the better your Electronics/Speakers are, the more they show differences in those recordings - you’re never Improving the signal with anything, you’ only making it less bad and I think once everyone understands that, then the differences between good and bad become a lot clearer! 
Not a bad metaphor, cbrents73. Just a side note to dig in a bit to the tubes/transistor point. You say one will be "more right." Each have qualities that are superior to the other. We know that traditionally transistor amps have killer, tight bass. Tubes have sweet, transparent mids and highs. Which is better? One is "right" for me, the other is "right" for you. Both, then, are "better" for someone, but they are certainly different. If one amp had the best qualities of both THAT would be the better amp. Otherwise you'll just have to choose which one you can live with; which one is right for you.
@dohanian - My solution to the tube/transistor, strength/weakness point, is to actively bi-amp, with SS on the bottom. The results can be wonderful, if it’s done correctly.