I have deleted the preferences as you suggested so the output port selection goes to Apple DLS, which works. If I select my USB keyboard as input, shut down and relaunch, this works without accessing the port selection dialog. So the problem seems to be entirely with output. As a matter of fact, I now see that with my usual port selections, the keyboard input *is* being seen by BIAB (judging from the upper keyboard animation, since there is nothing audible). It is only my external synths that get lost. I'm guessing that any software synth would work as well.

In case it matters, my setup is: USB keyboard as input, 2 hardware synths for output. The mixed audio outputs go to the computer line in. As stated before, when operating in the Core MIDI mode if, and only if, I hit the port selection 'ok' after launch, they are connected properly.