Digital=coaxial or optical in this case
Just run the optical output to your JB3. Mix the 4 mics with the level knobs on the front, the XLR inputs, and the Input Volume knob on the back--by the RCA inputs.
To use the setup you described, you will need an adapter cable to go from whatever the outputs on the Art Phantom box are to the RCAs on the UA5. I think that box has XLR outs so you need a dual Female XLR>Male RCA cable.