I can't test XLR output right now, but the SPDIF out will play YouTube etc, as you describe. Make sure the Eleven Rack is set "Use This Device for Sound Output" in Audio MIDI Setup. In my case, I also had to set "Digital Output" on the 11R to "Mirror Analog."

Actually - I did test the XLRs into my H4 recorder - they're outputting the Mac-generated sounds as well.
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