The job done by the mix engineer
is 90% of the process, and will certainly dictate how good the master can possibly be. Without a good mix to work with, the mastering engineer will be very limited in terms of how much he can improve the track. Audio mastering is not magic. A mastering engineer will not be able to take your crappy mix and magically transform it into a professional sounding record. If the mix sucks, the mastering will suck. An ME will really only be able to improve your track by one letter grade. So a C+ mix, will at best be a B+ master. It will never be the A+ master you'd like it to be. In order to achieve this, proper frequency balance is required. Once the mix engineer can nail this, the online mastering process will be easy sailing.