Yea, I agree. The bass sounds are the ones that always get me; especially if I save the bassline for last.
I usually use the same bass patch and tweak it, but when it really matters, and everything else in the beat is real real simple, then I carefully layer my bass patches, either with the same exact midi notes, or SLIGHTLY different timing. (As you know, you can't really play chords with bass notes, or they'll clash and your speakers'll fart.)
But if you take a deep sub-bass sound, and throw your lighter basses on top, and mix em REALLY low, then you get a much fuller sound. Also, try to transpose the notes on the other samples, and have different bass ranges at different octaves over each other.
And last but not least, take the bass sounds with the hard attacks and twangs, and filter them. Nothing sounds as good as a real bass guitar in the studio with the right compression and EQ, so if you filter some regular bass samples straight from the guitar, you can get some funky ass sounds.