Easiest route:
Go to your local musicians gear or RS store and buy an optical cable and adapter if necessary.
Alternative route:
First, get your UA5 levels where you want them with the SENS knobs, i.e. the "peak" light flashing, but not pegging, occasionally during loud sections. Then...according to the
UA5 manual:
9. Input monitor volume
This adjusts the volume at which the input signal from Input jacks 1/2, Input jacks
3/4, and the Digital input connector will be output to the Headphone jack and
Master output jacks. When this is turned all the way to the right, the volume will be
the same as the volume of the recorded signal.
11. Master Volume (labelled as Output on the UA5 face)
This adjusts the volume that is output from the Headphone jack and the Master
output jacks.
It seems the Input Monitor Volume knob and the Master Volume knob both work together to control the master output volume (i.e. the signal out of the UA5s rear RCA / 1/4" outputs).
So...try setting the Input Monitor Volume knob all the way clockwise, and then use the Master Volume (
Output) knob to control your output levels to your JB3.