Hello Kevin,
Since I am trying to export as MXF OP1A, I first looked out for different solutions premier, vegas, avid, etc. And when exported in all these softwares, only AVID exported with multiple mono tracks/streams, but both premier pro and vegas downmixed them into single tracks. Hence I decided to go ahead with AVID.
As per my knowledge, QT reference wont support multiple mono tracks. I once exported, in avid, as QT movie file with surround setup, I only got 1 audio stream, with 6 channels integrated in it which has digital dolby surround playback.
Sorry, I couldn’t help much as I have very little knowledge regarding codecs. Hope you find what you are looking for.