Over the years, generally speaking, any of the source media discussed here will sound good , IF, the recording and mixing process is done with care right at the beginning. Many cds of popular music are recorded and mixed at several different locations, hence, often the uneven quality that it is heard on that recording. More often than not Jazz and Classical music is superior because they are recorded at one location where the engineering process always seems to be done with extra care and you will see this style of music used more often when a reviewer is doing critiques of various equipment.
If one looks back in the history of recorded music, when mono changed to stereo, even then there was a reluctance on the part of producers and recording engineers to deal with it and that went through several incarnations before they figured out what to do with it and finally established some sort of standard. Mult-channel audio(SACD and DVD Audio) among others was even a bigger challenge that they really didn't want to deal with, thus, it really never caught hold and prices were too high anyway. A lot of this stuff remained in the "boutique" category because the major companies such as Sony, didn't want to spend the money to promote it and gave up early on in the game. "AIX" is an example of a company who is really interested in superior recordings and sound, BUT, once again, it won't be mainstream and they are dealing in certain genres of music and many unknown performers.
In the case of older movies transferred to Blu-Ray, it just seems to be a case if the original movie was, in its original form and quality, crappy to begin with, there is not much one can do with the transfer and, of course, the audio, is never going to be as good as a newer movie with its superior recording systems.
What is that saying, "you can't put lipstick on a pig"!
