If you are patching RCAs out of the SBD and into your UA-5, then the input volume knob on the rear of the UA-5 is your friend. You may definately use it to lower the db level. When I get a board feed, that's what I use to control levels.
I was told to run Lo-Z every time, no matter board feed or mic feed, if I'm wrong someone else please advise. I wouldn't want to tell him the wrong way to do it.