Sorry, I'm running the h320.
Ok - forget the advice about the optical cable...no dig in on the 320.
When you had you levels on the UA5 cranked...were you getting the peak lights to come on at all...?
You sort of want to get the UA5 levels set a certain way - regardless of what you connect to it...you surely dont want to use it to drive the gain stage on a downstream device...
Are you certain your batteries on the UA5 were up to snuff for the new mics? Starting to wonder about power at this point...
Using digital out, the UA5 (when getting low on power) will just conk out on the optical jack....but running analog might result in a slow decay in sound...and a high noise floor as your phantom faded to nothing...(and no peak lights)