Here's a trick you can do.
1. Take the bass sample, and assign it to a second pad (so it's on 2 pads.)
2. Pitch the sample down to -120 (one octave down) only on the second pad. If you want, you can also filter it down to 40 or so depending on the bass sound.
3. Go to the original bass pad's assign screen, and set the SIMUL function to trigger the pitched-down bass along with your original bass.
4. Go to the mixer screen, and lower the volume of the lower octave bass pad, until you don't hear two distinct notes when you hit the first bass pad.
If you set the volume just right, the lower octive bass will just be low rumble, and make the original bass sound fatter and lower.
5. As long as your first pad is set to NOTE OFF, you won't have to worry about the length of the lower pitched bass when you play your bass line.
This way, even when you're playing a higher note, you'll have some beef from an octave below that, so it won't sound whimpy when you play the high notes.
Try it, and let me know how it works out.
PS - If you're using 16 Levels of pitch to play your bass line, then keep the original bass at pitch "0", and set your low bass layer at "-120"
BUT, if you pitched the original sample to make it in key with the song, then you have to make the second bass pitched -120 below whatever you had the first bass set at.
For example, if you had your original bass set at +10, then your low layer bass will be at -110.
.....or if your original bass is set at -10, then set your second bass to -130.
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