A lot of audiophiles believe that mastering plays an absolutely decisive role in the sound of recorded music. I'm not sure how this belief came to be, it shortchanges all of the other elements of record production. It's akin to saying a chef can save a poorly prepared meal by making the food look nice on the plate.
That's just my opinion, so I rang up a mastering engineer friend of mine, Dave McNair, and put the question to him, "What are the biggest factors determining the sound of recorded music?" Without hesitating he chuckled and said, "Mastering matters the least of anything in the entire process."
So I followed up with what matters most? McNair, "Mixing is the biggest thing, it can transform a reasonably good production." Even if the band's performances weren't stellar, and the recording sounded just passable, a very talented mix engineer can make the music sound great. McNair added that's the way it is now, back in the analog days the recording engineer's skills could make or break the sound of the final product. McNair again, "Recording analog is a total pain in the ass, you have to work really hard just to overcome the limitations of the medium."
Up through the mid-to late 1980s the quality of the basic tracks was crucial because mixing tools were pretty limited -- there was equalization, compression and reverberation - that was it. In those days the recording engineers usually mixed the tunes. By the time automated consoles were introduced in the late 1980s mix engineers started to specialize.
Today's mixers can turn competent players into virtuosos; the engineer could, for example, create entirely new drum sounds that didn't exist during the recording sessions. Recording engineers no longer have to fret over getting just the right microphone for the vocals, because when the vocals are mixed they're going to be so massively manipulated, Auto Tuned, and spit out of a computer the quality of the original session tracks doesn't matter all that much.
McNair took a breath and said, "The best sounding recordings are the ones that have simple arrangements, well-played by the band in a great-sounding studio." In those cases the mixer's role is smaller, they just balance the tracks, add some nice reverb and you're done. If the sound was good to start with, and didn't require production tricks to help it along, the mix engineer will have less repair work to do. Taylor Swift and most pop music artists rely heavily on production to create their sound, the mix engineers make the magic happen.
When I asked McNair what are the "ingredients" of a great-sounding album, he said, "Starting with the most important, it's the songs, the musicians, the production, the mixer, the recording engineer, and in last place, the mastering." McNair's resume includes credits as a producer, mixer, recording and mastering engineer.
My take is great-sounding recordings have always been rare, but in 2015 the quality of the average rock album has gone way down, they are more compressed and crunchy sounding than ever before. I agree with McNair, and don't put the blame at the mastering engineers' feet, it's the total production that's responsible for the sound -- good or bad -- of music.
Dave McNair has worked with; David Bowie, Beck, The Derek Trucks Band, Maroon 5, Bob Dylan, Bruce Springsteen, Willie Nelson, Los Lobos, and many others.