UA-101 with Digital monitors

Hi,

I have started connecting my UA-101 to my Roland DS-8 monitors with toslink. What I didn't appreciate is that I have to raise/lower the volume using soft control. Consequently, I nearly blew up my speakers.

What I would like to know is how I can cut the monitors when I just want the headphone jack on front of the unit sending out sound. At the moment I have to enter soft control, revert to original settings, etc. Is there some kind of kill switch?

I have a feeling I am missing something here.