the RCA's on the back are looking for a line-level signal, which is what the board will be putting out. The problem might be that if the board is putting out +4 line level, and I believe the ua5 is looking for a -10 (i'm not sure on this), you could run into some overload there. I usually deal w/ Mackie boards, and the lower end versions have a "tape out" that is RCA, and I run out of that. Basicaly, it just depends on what the FOH gives you for a feed...