You might want to look at this:
The Amazing Slowdowner
http://www.ronimusic.com/
This will not isolate the bass part but it will transpose, slow down(I find it still useable at 50%speed) and loop a specific passage. Also you can save your changes as a new track. In other words if I need the song in A and it's in G I just transpose and save then add to my playlist and I am good to go.
Now you do lose some definition with the changes. If a part is complicated I may learn it in the original key. Then make a copy in the key I want to practice.
The slow down feature is great. I have gotten into the habit of going through my set list at about 75%. I will really tighten you up.
Best 50.00 I ever spent. I could not imagine doing my church gig without it.
Oh, and you can download a sample program to try it out.