Change audio output from main to spdif

I have a Lexicon u82s external audio device connected via USB and a set of JBL LSR speakers connected to the Lexicon decide with s/pdif. I am having trouble getting S/PDIF output for the "normal" audio out, e.g. from Safari or iTunes, while it works fine in Cubase, where I am able to change the channel from 1/2 (analogue) to 3/4 (S/PDIF). If I open Utilities - Audio MIDI Setup I am able to open the drop-down menu for my Lexicon device and chose Configure Speakers. When I test the speakers, changing from Main to S/PDIF, the audio is correctly routed to my JBL LSR. When I change from Main to S/PDIF for both speakers and save, which I believe would solve my problem, it automatically "jumps" back to Main, I am not able to change and save this new setting from Main to S/PDIF.

Any suggestions?