58,
I think that the changes the "mastering engineer" is suggesting is more of what the producer used to do. Guys like George Martin, Bones Howe, Jimmy Iovine, Tom Dowd, Mutt Lange and Phil Spector would guide the artists to produce the best song, the best mix, the best balance. From that would come the master tape. Of course, somebody sitting in their basement isn't going to get Jimmy Iovine to come by and adjust their mixes.
Mastering engineers in the original vinyl area would adjust the final two track master tape so that it fit into the parameters needed for cutting a lacquer on a lathe. These were guys like Stan Ricker, Bernie Grundman, Bob Katz and Robert Ludwig, and in many cases were uncredited unless you knew the codes in the lead out groove area. They were needed because tape masters had fewer limitations than vinyl records. You could put the bass on the left without a problem on a tape, but it wasn't a good idea to cut grooves that way. Cut too loud and you needed to have wider groove spaces, which means less time on the side. Cut too low, and the surface noise intrudes on the music. It was up to the mastering engineer to make the square peg fit into the round hole, so to speak.
I can't ever remember a story about a mastering engineer going back to Phil Spector or Mutt Lange and saying "you need to bring up the bass, and take down the drums. It needs more vocals." They probably would have gotten a mic stand shoved up their butt!
