When BIAB plays a MIDI trumpet part, it sounds like a trumpet.

How is this working, on a Mac?

As I understand things, BIAB is invoking CoreMIDI, which must be using some particular General MIDI sound bank, or sound font, or whatever it's called, correct? If so, where can I specify that? How could I use a different one?

In Logic, there is an instrument called "DLSMusicDevice" that uses Apple Sound Bank Synthesizer to point to a Sound Bank named "QuickTime Music Synthesizer". Is this what BIAB is doing also?

Yes, on Mac we recommend to use the "Apple: DLSMusicDevice" as the default synth.

To select this, in the mixer make sure you are on the “Plugins” tab. Click on your MIDI track and go to “Re-Route to MIDI Plugin”, and select “[Default Synth]: Apple: DLSMusicDevice” in the second menu.

If you're unable to see DLSMusicDevice in the plugin list, click on the Options button in the Plugins window, then click Exclude MIDI Plugins and uncheck <AUi> Apple: DLSMusicDevice.

Thanks, Chantelle, found it.