I run my sound through my bass amp when learning new stuff, it brings out the bass for me. I also hunt on YouTube to see if there's by chance a live video that *actually* shows enough of the bass player for me to see where they are playing. I run it through Adobe Audition if I need to slow it down.......I play it over and over and over and over until my dog begs me to stop.....and if I have to I go on a chord/tab chart hunt on the net. I also have taken songs to my teacher......only prob with that is a lot of times he just plays what works which may not be the "original" line and I end up figurin in out myself

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BTW, if you ever do Nirvana, I swear every recording they've done that I can find is out of tune (like a 1/8 tone off)...will drive you insane......
And sometimes.......I hate to admit this......the guitar player helps me figure it out. I also have the Bass White Pages book, although I've only ever used it for a song or two (and ended up doin my own thing anyhow!).
Something I have realized from learning cover tunes is a lot of times, the bass player has played this line a billion times. To keep themselves occupied, they throw octaves and stuff in that may not be in the original. I don't worry about every one of those notes, just in the important spots. And now I find myself throwing those little chromatic walks and dead notes in when I get bored LOL........