If no upsampling or some kind of DSP is involved, there is no “SQ” to the software players, they are all bit-perfect and therefore sound the same. If not, and there is no user misconfiguration, it is a bad player and should be avoided without any further discussion.
Things are of course made more confusing by companies claiming marketing nonsense like “More transparent than ever” while at the same time advertising “bit perfect”.
At leas on Mac, there is also nothing to “optimize”. For inexperienced user, software can help to configure things that can be done manually, but that’s it. iTunes can play bit perfect as any other software player.
For the USB transfer, DAC is the “master” and there is nothing to improve, unless DAC has design flaws. If there are any issues, better buy DDC of some kind and that will be a much better investment. So snake oil again.
When things like library organization, DSP, digital volume control, upsampling and such are considered, this is where the real “heavy stuff” of player design comes into play, and that is what you pay for and that is where the honest marketing should focus on.
But that is not as attractive as simple snake oil advertising like “Pristine sound” which means nothing but attracts people and make them imagine things that are not there.
Finally, there are unverified (but with some screenshots in French) claims on the internet that Audirvana support has claimed that A+ actually adds some “secret sauce processing”. I find it difficult to believe because given the claims of A+ being bit perfect it would be a clear false advertising. But if true, it could explain why too many believe that A+ sounded “better”. It could potentially also somewhat explain why AS might not sound “as good”.