Can I use Symphony Desktop as a standalone unit or DAC?

Yes, if you are using current Touchscreen Control firmware (use at least version 1.20.8), you can.

All parameters, with the exception of sample-rate, can be adjusted from the Symphony Desktop's Touchscreen Control. If you need to adjust the sample-rate, you will have to connect the Symphony Desktop to your Mac or PC first to make the sample-rate adjustment, but any sample-rate setting is saved and recalled on the Symphony Desktop across power-cycles in Standalone mode.