One interesting thing that I learned about the outboard pre/DI (which [Invalid or Expired Link Removed] after a while) is that the volume is actually an attenuator.
The "boost" of the preamp comes from the treble and bass controls while the volume knob just attenuates the signal from your instrument.
So, if you've got a passive bass, you likely want the volume all the way on. However, if you've got a hot bass or have to use one that is hot due to an onboard pre, you can turn the volume down so that the signal doesn't distort.
Once you've got the volume/level worked out so that it's not distorting, set the tone and bass knobs all the way off—which is the flat setting—and then adjust up from there to taste.