[quote name='Roland Rock' timestamp='1444802172' post='2886075']
If I need to learn a song, I'll usually do it by ear. I found that being familiar with chord tones and chord progressions helps a lot - it makes it easier to anticipate what will come next, and that understanding also helps you to play around with the line a bit later on.
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This. The first thing I try to determine is the initial key and any modulation from it further on in the song. That will help me predict what chords to expect, although it's also wise to keep in mind the occurring of passing notes, chromatic fills, etc.
Listening to the highest possible quality recording/MP3, with the highest possible quality headphones, will help too, of course.
Having said that, there will be sections where the bass is simply inaudible or so muddy you can't make it out. In that case, if you know what key you're supposed to be in, you can provide your own suitable bassline. It's great fun