IMGO : the Line6 Audio MIDI preference in the system preferences should do the job.
It does not.
So, I use the *Audio Hijack* application (see attached screen capture).
1) I create a blank session with a name such as *Transcribe -> HD500*
2) I set an input block for my audio App (DAW or Transcribe or iTunes or whatever)
3) I set an output block for my audio device HD500
4) I set the volume for that device to a very low value, say 1%
When I click *Run* the audio is routed to my HD500 with a much lower volume, and I can adjust the volume is my audio App
When I click Stop, the audio is routed back to my system audio output (internal speaker, etc.) with the usual volume
I have one session per audio App.
No disturbing system sounds, etc.
No more loudness in my ears by accident.
Have fun !
