bitflipper
Can you elaborate on the term "linear sound"? Are you after longer sustains? 
 
A sampled stringed instrument is going to naturally exhibit non-linear decays, because that's the nature of vibrating strings. "Linearity" isn't a word I normally associate with sampled instruments.
 
Or are you referring to spectral distribution, perhaps seeking an instrument that emphasizes low frequencies? Usually, we're struggling to achieve consistent levels on basses. I'd be happy with a bass that was linear in terms of levels, but that's rare among sample libraries.
 
I noticed that in your sample that bass has a lot of distortion on it for such a mellow tune, but it's rolled off on the high notes. You can do that kind of thing with any clean DI bass samples by adding an amp sim or distortion plugin. The distortion helps low notes cut through the mix, but is less-pronounced on high notes. Definitely a "non-linear" effect. I use Voxengo Boogex for that on bass. It's a free download, give it a try.

listen https://www.youtube.com/watch?v=7nETgHogKi4 Minuts 1:38 from now on
The bass never wanes

As a bass player, what your hearing is a bass with great sustain and played by a great player. Not all basses are the same. I had an Ibanez bass at one time that was great for slapping and popping but had no sustain. The Fender I have now has pretty good sustain but not like the older Precision bass I used to play about a 100 years ago in the early 1980s. There is no special plug ins that will help unless you have something special to begin with.

Consider using a sampled or synthesized bass, tyros5. It's easier to control the attack and sustain on a sampled/synthesized bass than a real one. If you want a real bass to sustain longer, use new strings, turn it up louder, and hit the strings softer. If that's not enough sustain, try to hit the strings about half-way between the fretted note and the bridge.