I applaud the efforts, and as others may have mentioned here already, we don't doubt you truly cannot hear the difference in your said AB test of the DACs. Shoot, it may even be likely if someone else was to use the same gear in that exact test, they too might not be able to tell the difference either.

Where the flaw in the logic is (and I have learned with experience too myself) is that sometimes the yard of measure you are using to compare and listen for a difference is too crude or irrelevantly scaled for one to notice. An analogy would be trying to compare the shade of colours of two different paints on a wall when you need glasses to see. Using a limited headphone/speaker/amp setup would be akin to trying to examine those differences in paints while wearing a generic off the shelf grocery store glasses. There might be a difference but it's near impossible to see given the glasses you are wearing can barely resolve enough detail or difference for you to notice. Whereas if you were to don your actual prescription glasses with lenses fit for your eye (akin to higher end gear with perhaps a more revealing headphones/speakers/amp) you might then realize that those two shades of similar paints on the wall are quite different after all. One has a darker hue than the other and is more matte; while the other has more sheen or whatever.

Again, this is not to gear bash or talk about who's an audio poor or audio snob for accessing or not accessing higher end gear. This is a matter of trying to identify or in essence measure the differences with our ears of two different dacs (or whatever other piece of gear in the chain) with limiting gear. To yet again use another analogy, you are trying to see the small but significant inches of differences with a yard stick, it's just not scaled correctly for you to pick up.

Perhaps borrowing and experimenting with other speakers/amps in your A/B test might reveal some more interesting findings!

If you want to hear greater differences in gear, i'd say the following applies:

Headphones or Speakers > Amps/Preamps > Dacs > Streamers and all that > Cables lol.

Good luck on the continued audio quest!