In my view, there are two potentially-informative ways to test MQA's sound quality:
1) One system with an MQA DAC--
(a) Play a non-MQA digital download or stream of a song;
(b) Play the MQA version of the same song.
*If the non-MQA version was streamed, then stream the MQA version. If the non-MQA was downloaded, then... well, I'm sure you all get the gist.
*The only variable changed in this test is the version of file played, non-MQA or MQA.
*This test is to ascertain what, if any, differences there are between the MQA file and its non-MQA version.
2) One system with an MQA DAC and a non-MQA DAC, same brand and model except for the MQA capability.
(a) Play a non-MQA download or stream through the MQA DAC.
(b) Play the same version through the non-MQA DAC.
*The only variable changed here is the DAC.
*This test is to ascertain what, if any, differences an MQA DAC presents compared to the non-MQA version of the same DAC.