WORLD-CLASS ANALOG MASTERING FOR THE PEOPLE
TOP 3 TIPS WHEN SUBMITTING AUDIO FOR MASTERING
Sending 24-bit (or 32-bit) WAV files is ideal for mastering because these are uncompressed files and none of your data has been truncated. 16-bit WAV are not as good (since the data has been dithered and truncated). MP3 files are generally the worst files to submit (since they have been heavily compressed). However, we can usually work with MP3's if that's all you have but its not ideal.
3) Send mixes that have not already been "pre-mastered" or maximized for loudness using plugins
2) Send mixes that are not clipping above 0dBFS (i.e. no digital clipping)
1) Send mixes that are in WAV format (24-bit or 32-bit)
Audio is a wave with peaks and valleys. Once a peak hits 0dBFS in your DAW it becomes digitally clipped. This results in damage to the audio that can’t be undone. The easiest way to prevent this is to just turn down the master fader before rendering the file so the peaks are less than 0dBFS (i.e. more headroom). How low you want to go doesn't really matter. The main thing is that the peaks should not be hitting 0dBFS.
Louder is not always better. If the mixes you send are already really loud, chances are they have been processed using digital peak-limiters and/or clippers (i.e. the audio has been damaged to a certain extent). This will usually hinder my ability to properly enhance your music and deliver quality masters (since the quality of the source audio has already been degraded).