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
The cupped-hands technique definitely yields a "sea-shell" effect.
Closing the eyes always improves aural acuity.
Removing eye-glasses does too.
Still trying to prove to my wife I'm semi-sane also...
Act chew ally the cupped hands improves dynamic range, especially for systems that lack dynamic range, and loudness, i.e., a better antenna, but the down side is it impacts soundstage. Once your system can perform dynamically, e.g., room acoustics treatments, you’ll discover the cupped hands doesn’t do much at all. Recordings are made with human ears in mind, not Mr. Spock’s. You’re not hearing the rear reflections since the hands block them.
In fact, in the case of power amplifiers as an example; it needs to accurately amplify the input signal, and transfer that signal, fully intact, to a low impedance load.
A well-designed power amplifier, in theory, is not supposed to reshape the input in any unintended or involuntary manner, so I support the concept that accuracy improves when the potential for error and distortion is minimized. 

Hence there are indeed, subjective differences in DAC,s and particularly pre/ power amps - however: much depends on how they integrate with the speaker load and the components having low errors and distortion.
see: https://www.thesoundadvocate.com/2018/10/the-role-of-power-amplifiers-today/
Unfortunately, there are many people who’ve never been taught how to listen or what to listen for and that’s because they’ve never met a person, or found a dealer/store, who was willing or able to teach them.  Can you tell the difference between a male and female voice?  Congratulations, you can hear.  If you go to a Hifi shop to listen with the preconceived notion you won’t hear any differences, then there’s a great chance you won’t.  The teacher has to be willing to teach, but the student also has to be willing to learn...