I have taught College and University ear training at a variety of schools. I have come to the conclusion that the most important parts of my training, the parts that made everything else happen for me as a musician were:
1) my parents teaching me to sing songs as a very little kid.
2) childrens choir ages 7-11
3) figuring out every single bass line on the first 6 beatles LPs as a teenager.
My advice is to try everything to improve your ears (lessons, software, classes, piano lessons, etc.), BUT....Do Not avoid doing the very thing you are trying to do.
In other words, if you want to get better at learning bass lines from recordings, work at learning bass lines as possible. yep. Learn lots of bass lines. Start simple, and do a whole lot of it. Do it everyday, and do it for an hour or two.
Once I got pretty good, I used to just put AM ('60s) top 40 radio on and play along for hours.
Do it. Start with simpler recordings, and move up. You'll get better, I promise.
