Why are you running the U5 into the RNP? My understanding (though I haven't used one in a few months) is that the U5 will put out line level.
Ah - here's what I was looking for: the output of the avalon is "+30dBu DC coupled, balanced 600 ohms (mic and line out)". Make sure that if you're using the RMP that you're using the mic out, not the line out. You should be able to take the line out directly to the MOTU interface.
Besides checkint the batteries in the bass, it sounds to me like you've got some sort of major mis-match with the signal chain.