OK, flame-retardant suit on before the “Bob Stuart is God-inventor of all things digital” crowd come banging.
There’s one serious study that I know of, and that came to the conclusion that MQA, in itself, was not appreciably different to the test subjects.
Problem is, MQA’d releases are also often remasters, and that’s obviously audible. It’s extremely likely that there’s orders of magnitude more differences between masters than between even wildly varying in quality, but competently designed, dacs. It’s likely that different remasters is what you might be hearing, more than anything else.
It’s also possible that it’s audible on some gear, more than other, and/or that it’s symbiotic, rather than parasitic, with some brands, or that the distortions that might be introduced by the process might sound good with people who like certain types of distortion (and therefore already bought in to other things designed by BS).
This might be of interest as to the differences between marketing and reality as far as MQA is concerned.