Set the UA flat, turn your bass volume and tone all the way up and start from there. When you get the tone you want, then roll off the bass tone knob to take off some of the edge. Starting out with the bass tone control off, you are essentially putting a sock in the UA's mouth and reducing the instruments level output somewhat.
The UA takes some tweaking but when you get used to it, you will really dig it. You will want to adjust the output gain so the volume is the same with the pedal on or off, when dealing with other effects in the chain.