Over the years I've been allowed to compare various brands including Theta, Wadia, Mark Levinson, and more recently Benchmark. These all sounded fine to my ears, but nothing has ever leapt out at me, even after extended periods of listening. I've heard some beautiful systems in showrooms, and was even allowed to bring in and hook up my cheap front end. Still sounded fantastic. I've lost track of all the different types I've heard at shows and in various showrooms. I've never done controlled blind tests because I never had a claim of a difference to make. I don't know of any controlled tests that have confirmed anything and that doesn't surprise me. Of all the systems I've heard and owned over the years, there seems to be no correlation between the dac and the sound quality. Some of the best sound I've heard at shows involved self powered speakers with their own dacs, or in one case a dongle dac from a Macbook air. I've noticed that even in the 6 figure systems, I still hear the basic issues with the standard 2 speaker configuration in rectangular rooms dominating the overall presentation.
I'm not going to make a general statement that nobody can hear differences. I'm just saying I can hear a lot of stuff but I can't hear a difference between reasonably competent dacs. If people can hear a difference it just means they are very tuned in to specific aspects of the sound, while able to ignore or adapt to other, louder issues. Decent dacs at all price ranges perform extremely well, and very close to the same. That's not the case with speakers and amps, which are made for all sorts of different listening situations, so they have different power and bandwidth ratings. They also just plain differ a lot in their response characteristics.
Speakers and amp combinations and rooms and speaker configurations - those make very significant differences to me. Some pre-amps seem to do weird things too. If I go way back to the early 1990s I recall that a Theta dac had more oomph than the analog out of my Sony ES CD player, but that could have just been level matching. I bought it and enjoyed it. 15 years later I still had the dac and compared it to the analog out of a new cheap DVD player and it had nothing on the DVD player. Hard to say why I thought I heard something earlier but I could see how in some cases you could have an output/input mismatch between a dac analog stage and pre-amp input stage.
Cases where I have heard a difference is things like hooking up a pro-level dac to a consumer level pre. Output is too hot. I then modified the output by bypassing the op-amps and using a cap and a resistor. That's not to spec, and that had audible problems, but even then, not enough to make or break compared to other factors. I was forced to use digital attenuation, which seemed to clear up unpleasant audible issues but then I lost headroom and max volume. I've also tried some crazy output filterless and non-upsampling dacs. Apparently my downstream equipment didn't have a problem with the signal and I can't hear past 20kHz so nothing about it bothered me, which I thought was interesting, supporting the idea that dacs are generally better than they really need to be.