Have you tried plugging the bass into channel #1 or #2 so that you can use the preamp on the soundcard.
I looked into the
M-Audio 1814. It appears that it only has 2 preamps.
I've got a feeling there is an impedance mismatch between your bass and your line inputs on your 1814. Try pluggin the bass into channel #1 and boosting with the preamp to see what happens. If this doesn't fix it, a direct box would be the next step.
Have you tried plugging an electric guitar (like a Les Paul or Strat or whatever) into 1814? How did it do?
Brandon