Layer a sub bass underneath it ( pure sine ) Make sure the sampled bass doesn't generate any sub, so use a HP filter at a cut off point where the sine wave is providing the bottom end. That should give you the weight.
Now for a an extra bit of presence and fatness, make a copy of the sampled bass and apply some chorus and maybe a bit of distortion, the chorus will give some width and the distortion will add extra harmonics, HP filter this and pull the channel fader right down. Now edge it up slowly alongside your sine/sampled bass until you just start to hear it having an effect.
I always think bass sounds fatter when the main bottom end is in mono and free of any effects. If you want to add a bit of 'shimmer' or with chorusing or distortion or whatever, HP filter it at at least 100hz and balance the levels of the 2 signals until you get the required sound, the maybe set up a buss and compress the 2 signals together. The reason why the effected copy of the bass is HP filtered is that this leaves the low end completely clean and a much more solid sound.
Try it, works great.