Just for funzies, here are two samples that are the same bass, but just set up differently. You can hear how one is very rounded and soft. The other is very aggressive and growly/grindy. It would be more so through an amp with a little overdrive I think. This is DI'd straight in with no processing. Not say the grindy one is necessarily a perfect example of growl, but just to show how much setup and playing style affects the tone.
The normal tone: http://www.charmedquark.com/Web2/TmpAudio/REDDBass2.Wav
The grindy tone (listen to the whole thing, it's not long. It's more so towards the end): http://www.charmedquark.com/Web2/TmpAudio/REDDBass3.Wav
One is a higher string height setup, and played in a more 'parallel to the fret board' sort of way, which minimizes any contact with the frets. The other is a low string height setup and is bring played more 'down through the string' to force maximum contact with the frets. You'd think that the latter would be very, very buzzy, but in fact what happens is that it smacks into the frets and settles very quickly. You get an initial nice buzz, but it damps down almost like a compressor but totally naturally.
You can also hear how there's much more upper lows/lower mids in the more traditional sound, and it actually sort of blooms up sometimes as the string vibrates up to its maximum travel. The more aggressively played one sounds actually fairly scooped out. The string never 'spins up' and gets to vibrating very much. A little of that is pickup selection but not that much. Both are being plucked in about the same position about over the kneck pickup, so it's not hand position doing it either.