Listening to cellists etc. is all well and good, but who knows whether your playing will actually benefit from it.
Here's a way to make sure it will though:
Take a wonderful recording made by a bassist you want to sound like. Not a very 'hard' piece, choose something which you'll actually be able to get around. Preferably something beautiful and inspiring. And.. go about copying his playing.
Find his sound on your own bass, listen and copy his vibrato, his phrasing.. pay attention to details like when and how he changes that sound and that vibrato as he phrases, listen to how he shifts and how much of that is heard and enjoyable to hear, really get into his mind and body to figure out how, on a 'technical' level, he goes about making each phrase beautiful.. and do it yourself! Record yourself to see how different you sound, what you are not yet doing, and what you are possibly doing which you shouldn't be.. and play with the recording also to understand why each note is where it is, not a bit earlier or later. Be patient and stick with it until you've digested all that's in there and can reproduce that.
It is of course very hard to do this and not everyone can. It takes great ingenuity. If you can do it however, you will learn a lot from it.