Think of an animal growling, like a dog or tiger, and that note sustains in a pure form. I always think of the pure acoustic sound a brand new round wound string makes, the beauty and simplicity of that vibrating, oscillating metal, and the hopes that that pure and perfect sound will transfer through the pickup, out the cable, into the amp, and be uncompromised and retain all that sweet pure growl when it comes out the speaker.
Achieving it? It's like the planets aligning, my friend. Some basses lend themselves to a growl naturally, others not so much. You hope you have the right combination of bright tone-ful string in the proper gauge, resonate wood, and pickup wound just so. How well the bass is setup and pickup height probably matter quite a bit. I'm not even sure how much the amp factors in. This is something that you want to come primarily from the guitar. I can tell a growly bass through my cheapest practice amps, and sometimes with no amp at all.
Of course, that Ric in the video is a prime candidate. It growls, but I also hear a lot of tube warmth, which is not growl, but doesn't hurt one bit in my opinion.
