This approach usually works for me.
1. start with a simple sine wave generator.
2. Use a bass processor that will do something like a. double the octave, b. invert the wave-form, or c. add distortion. I know there's a free VST plugin made by MDA that does a nice job.
3. Add a small touch of chorus or some short multi-tap delays, and your tone should shake the roof off.
Peace,
Matt