Your stereo mic should have a stereo output capable of feeding both L and R to the UA5. You need to split your single-connector stereo-output XLR to dual XLR stereo-output (basically, a Y cable, but a stereo on the single-connector side instead of mono). If you're running the UA5, why not take advantage of its superior ADC relative to the JB3? IMO it's easier to set levels with the UA5 than the JB3. Easier, sounds better, go for it.