When layering in a synth sub bass, often a sine wave is used because it's the most basic and purest wave form. It's the root of all sound, a digital creation that doesn't occure like that in "real nature".
It's exactly the frequency we want to make stronger (and not a certain area with a certain slope, like an EQ), with nothing else in there that would make the sound distinct - no erratic dynamic changes, no harmonics, etc. We don't want to actually make the double basses sound like something else, we're just empowering the fundamentals. The sine wave is just droning underneath, with no distinct character of its own, nothing that gives its presence away immediately. It's basically adding something that is hard to locate and single out, but at the same time, cannot be unheard.
That way, it blends with the basses perfectly, and the result is a kind of a supernatural Double Bass. Because the digital sine wave it's a perfectly even, artificial sound, it fastens the overall sound and makes it firm and sleek. It evens out the evolving and ever-changing sound of bowed strings. Almost like a compression effect, but without compromizing the dynamics of our "real" basses in any way.
This of course has the effect hat the basses assume a more even, cold, monolithic and "modern" artificial sound. Which can be great for hybrid and electronic-infused music, but terrible for an impression of a "natural" orchestra, and in this case, thickening the basses with the method that Beat suggested (layering in basses of another library) works much better.
Layering has a different effect than simply pushing frequencies with the EQ, because by doing so, you would be "selectively turning the volume up". You would be shifting the spectral balance of something that's already there. But actually adding another voice into the mix obviously makes the whole arrangement bigger. We're not just turning up, we're adding something more.
The effect of increasing size and thickness isn't just because of volume. It comes from the subtle differences in pitch, tone and dynamics. There's a certain friction and rub between the two similar sound sources that our hearing interprets as wider, thicker, more complex and powerful. They both play the same note, but there's always little things in there that clash and rub and create a texture that wouldn't be achievable by just one voice alone, and that somehow is pleasing to our ears.
It's an effect that occurs and is used in music all the time and in several ways. For example, when you listen to brass sections: a certain part of their power and vigor comes from the subtle variances in how the single players hit their target pitch.
That's also the reason why we double- or quad-track guitars for a "wall ouf sound" in pop and rock productions. Using the same recording twice would sound like one track, but turned up louder. Just EQing the track would sound like - a filtered recording. By actually having the player record their parts 2 or 4 times and panning the tracks differently produces this thick, wonderfully juicy satisfying sound, because no matter how accomplished the player - every take will sound ever so slightly different and that's exactly the magic we're after.
That's also the reason why chorus effect units "thicken up" guitar or synth sounds - they create a copy of the signal that is mixed to the original, detuned and delayed slightly.
