-As mentioned earlier, how the mp3 was prepared, whether split or joint stereo encoding.
-The source of the instrumental version. If the source is vinyl, all bets are off. If the source is a proper "Instrumental Version", it would have been mastered from an identical mix but without vocals and vocal effects (ex. reverb), and with the exact same settings as the original master. That is how major labels typically do it --- if it's apparent that the album will be popular, they'll usually have an instrumental mix (and master) created at the same time. This is done because the label wants to have instrumental versions for later potential use in films, TV shows, award ceremonies, commercials, etc. It's much more expensive to go back and "recall" settings in order to "mix-down" instrumental versions. This usually requires hiring the original mix engineer or one of their assistants and renting the studio at which it was mixed. Many pieces of "analog" mixing gear are not "stepped" with repeatable settings. Analog gear also has differences between individual pieces of equipment, even if the same model. Equipment breaks, is sold or removed, and this makes the task of a recall that much more difficult. Recalling mastering settings isn't nearly as daunting, as mastering studios typically retain all of their equipment and keep it in working order. This means that good documentation is required, similar to the software development process. For many of your all-tim favorite albums, there exists binders full of charts, diagrams, and notes detailing microphone types and positioning in the room and relative to the instrument/speaker, channel assignments, mixing equipment settings, the type/speed/bias/etc. of tape, lyrics and notation, even the types and tuning of instruments. It's a very complex process to document and recreate.
-As mentioned earlier, how the mp3 was prepared, whether split or joint stereo encoding.
-The source of the instrumental version. If the source is vinyl, all bets are off. If the source is a proper "Instrumental Version", it would have been mastered from an identical mix but without vocals and vocal effects (ex. reverb), and with the exact same settings as the original master. That is how major labels typically do it --- if it's apparent that the album will be popular, they'll usually have an instrumental mix (and master) created at the same time. This is done because the label wants to have instrumental versions for later potential use in films, TV shows, award ceremonies, commercials, etc. It's much more expensive to go back and "recall" settings in order to "mix-down" instrumental versions. This usually requires hiring the original mix engineer or one of their assistants and renting the studio at which it was mixed. Many pieces of "analog" mixing gear are not "stepped" with repeatable settings. Analog gear also has differences between individual pieces of equipment, even if the same model. Equipment breaks, is sold or removed, and this makes the task of a recall that much more difficult. Recalling mastering settings isn't nearly as daunting, as mastering studios typically retain all of their equipment and keep it in working order. This means that good documentation is required, similar to the software development process. For many of your all-tim favorite albums, there exists binders full of charts, diagrams, and notes detailing microphone types and positioning in the room and relative to the instrument/speaker, channel assignments, mixing equipment settings, the type/speed/bias/etc. of tape, lyrics and notation, even the types and tuning of instruments. It's a very complex process to document and recreate.