I’ve at times struggled to hear a difference as well. I had Adio gd and various Squeezebox that I really couldn’t tell any difference while switching between them.
I recently purchased a Modwright Tranporter which I can clearly hear as being better than my stock dac in my 2 channel receiver. However, I would describe the difference as being more pleasurable above all else. It’s hard to put your finger on it in an analytical sense. The change is in the realm of more presence and better soundstage.
+1 on your type of listening and evaluating, and your conclusions.
For me, quickie a/b comparisons will (try) to detect frequency response differences, which among many dacs will be inaudible.
However, true timbre, touch, and texture, apparent soundstage (even in silence), and micro dynamic swings are so obvious when listening to music with, for example, Metrum, Holo, or Yggdrasil that I don't work at all to hear it. And when those characteristics are significantly reduced, it's just as obvious.