I use a lot of outboard gear and I also sum my mix through a passive summing mixer and preamp combination. When updating the mastering file, I sometimes get an audio engine dropout or CPU spike because my sessions tend to be quite large. To avoid this issue, I record a summed copy of my mix to one stereo track and then I have to export the file for mastering. Once the song is added to the mastering session, it would be great to be able to update the mastering file by having the option to either select the entire mix or the individual track that I've already rendered within my song session.