| Gary Willis has one of the best ear training books for bass players out there. He employs intervals for his particular method, not only ascending intervals, such as root to minor third, but descending intervals such as perfect fifth to minor third. for example. You need to be able to hear and identify intervals both ways.
He starts very simply then builds to ever greater complexity. If you procede slowly and diligently through this book and its CD, you should develop a much better understanding of intervals and recognition of them in music you hear or are trying to transcribe.
The book is: "Ultimate Ear Training for Guitar and Bass" with CD
He has another very helpful book, too, that will also carry you to the enxt level: "Fingerboard Harmony for Bass Guitar" with CD
The only caveat I would suggest with either book is that Willis plays his samples and drills rather fast. He probably thinks they are slow, but he is a seasoned veteran. Inexperienced bass players will have to work patiently at the exercises, but if they keep trying, they will be far better off for having done so, even if each book takes a year or more to master.
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"Jazz sounds like a very good blues band that fell down a flight of stairs."
Michael Buble, Canadian standards singer | 
