Instant Hijack and Digi CoreAudio driver are not friends
Just in case anyone else runs into the same issue…
I just spent about half an hour trying to figure out why I couldn't get the Digi CoreAudio driver to work. It knew that in order to grab the audio from a given application, one needs to launch that application after the CoreAudio Manager is connected to the hardware.
This whole "re-launch the application to grab the audio" thing reminded me of Audio Hijack/Airfoil without Instant Hijack installed, so it occurred to me that maybe Instant Hijack inserting itself between my applications and the Digi driver.
Turns out I was right: I uninstalled Instant Hijack (a quick and easy process, I might add), and when I logged back in, the CoreAudio manager was able to "attach clients".
I certainly can't blame either of these companies — they're both trying to do some rather unsupported stuff with system audio — but I figured I post in case anyone else runs into a similar issue.