As someone learning now what I should have learned earlier, the meat of this stuff is chord tones. The root, 3rd, 5th, 7th, and so on in a particular key, and the chords selected for a particular song, be it a major, minor, dominant, augmented...the stuff bass lines are made of consisting of the chords the other instrumentalists are playing together, but that we as bassists usually breakdown into a walking or syncopated series of single notes depending upon the flavor of the song. I think of scales as a secondary template over the chords when there is space to fill, or for soloing. Important, but not quite as much as understanding intervals and patterns moving in and out of them.
When I started first learning scales, it was frustrating learning to fly through a sequence of notes without a discernible application. But when I was introduced to the idea of connecting certain combinations of intervals within chords that form the core of most music, I started thinking of bass in its foundational role, and not as just another instrument doing its own thing at lower frequencies. I realized this concept (for my brain) had to establish itself before I concerned myself with the muscle memory of repetitious scales and modal theory. They are important, but to me, they seemed abstract until I nailed down the what it means to flat a 3rd and a 7th, and how to connect these intervals across the fretboard in a seamless manner. I’m still a work in progress, but I think now I’m headed in the right direction.
I enjoy listening to music through my headphones and aux cable to my iPhone and challenging myself to (1) establish the key, and (2) figure out the chords and play my own bass line, and not just someone else’s tablature. I won’t look up the chords or tabs until I’ve exhausted all efforts to figure it out on my own. Sometimes I’m close to the original recording, sometimes not, but it’s a great exercise for training my ear and my head to recognize what others are playing and how to make everything else sound good utilizing certain notes that will always work. After I feel good about that, I think about embellishing a little with scales and chromatic notes, or often, trimming back and adding rests when I think I’m getting busy fingers. But the good stuff comes faster when I’m learning and listening at the same time. Otherwise, I practice scales, get bored, start noodling, and time vaporizes.