What is this 'Growl' I Keep Hearing About?
"Growl" is usually described as a rumbling undertone to the note that makes it sound a little more guttural or aggressive; commonly heard in modern jazz, it's just a tonal color that some players find adds interest to the sound. It can mean different things to different people; in an informal survey that I put out asking experienced players how they'd describe it, I got several answers, most of which are pretty much along similar lines, but with some subtle variations.
Most players seem to agree that it's a "blooming" tone, involving a resonant sustain that can be somewhat controlled by the player - if you "work it" with your left hand (with a vibrato movement) you can keep it going. What does "bloom" mean? Simply, that the sound or intensity of the note seems to grow before it begins to decay. Others also invoke descriptions like, "gritty, sustaining, clear lows," or a "raspy" tone in the lower range of the instrument.
Many bassists, including myself, feel that it's partly caused by subtle interaction with the fingerboard, so low-ish action will help accommodate it -- but not so low that the strings "buzz." We assume that it occurs because lower action allows vibrating strings to very slightly "graze" the fingerboard near the finger, causing a slight overtone/distortion of the timbre. Action that is too high would eliminate the effect, clearly; there is a happy "medium" zone where you can find "growl." In my estimation, growl is sort of tangentially related to the "mwah" sound that you hear on electric fretless bass when the action is low.
Ultimately, it's kind of one of those things that is "you'll know it when you hear it." It is, to many contemporary styles, considered "aurally appealing" – though I'd wager that someone playing pizz sections in an orchestral piece would not be trying to create growl.
How do I get growl?
Bringing out that special growly tone has as much, if not more, to do with technique as it does with equipment; you'll find that playing with different levels of dynamics, and using different parts of your fingers (on both hands!) will affect how "growly" your tone can be. But certainly, certain strings (and setup, as mentioned above) can help achieve it, if you're looking to get that tone.
Lighter strings can sometimes create more "growl" as the lighter overall tension tends to lower the action as the neck "relaxes" a bit, and lighter strings vibrate more widely, which causes them to interact with the fingerboard a bit more. But keep in mind that setup and tone color of every bass is different, and some basses may be more capable of that sort of tone than others. The only way to know for certain is to try.
Basically, everyone's journey is personal, and there's no "right" string -- or tone -- for everyone.