I've done a few of matrix tapes with a UA5. I've only encountered delay issues after about 25'...
I've found the best for me was to run my AUD levels slightly low and fill them in with the SBD feed. I'd have the RCA input volume knob set at about "9-10o'clock". If you want to check the levels of each, you could turn off your phantom to check the SBD levels. Set to what you want. Then remembering this level, turn this all the way down, turn on phantom, adjust level. After adjust of AUD level, turn SBD back to the level you had figured out before.
Tell you the truth, I've never liked the matrix tapes I've made. To me they sound unnatural. Its like the band is right there playing to me and the rest of the audience is behind a glass window cheering away. But I suppose it about what you are used ot hearing... Me, I like straight AUD tapes...
Terry