I'm sorry if there was a misunderstanding. Of course the quicktime instrument is useful to play back the .mid file, but then you only have the whole mix, not the single instruments. I meant there is no Audio Multi-Instrument in logic that lets you load a .mid file of a whole song and assigns the different regions/channels to the instruments that they were intended for. For example, a region with a bassline would automatically played back with an AU that creates a bass sound.