>>Midi data is lost when you change to an audio port?
If you switch back to midi, the data is still there.
If you're talking about converting a midi track to an audio aiff, why do you need the midi data in it? You can always record to a different track and keep the original data.
The concept of the aux bus is to change the MAIN popup to AUX1 in the Mixer window and turn up the faders on the tracks you want it allocated to.
For instance........Sampletank sitting on the aux bus has 16 channels.
You can allocate piano to 1, bass to 2, guitar to 3 etc. inside sampletank's window.
Change from internal to sampletank in the mixer and turn the main volumes down on the tracks and the auxess up. You will then only hear the aux sound.........and like you say only the aux bus should synthesize.