I use software called Slowgold. You can check out a free demo at
http://www.slowgold.com/
I used the demo for one hour and my ears were convinced.
The program can slow down or speed up any music from a CD or .wav file Without changing the pitch.
Or it can change the pitch up or down as much as a full octave without changing the tempo.
Or it can do both at the same time.
For bass transcriptions, this is what I do:
I have Slowgold record a portion of a track from a CD. Then I have Slowgold slow it down to about 70% of the original tempo and raise the pitch one full octave. This way I can hear the bass parts more clearly as they stand out better at the higher pitch. Also, the low bass sounds are really at the low end of the spectrum of human hearing. By raising the pitch a full octave, your ears are now more sensitive to the raised bass frequencies and you can discern them much better.
I also use two separate dual-15 band EQs. I chain the left and right channels of one EQ together to get 24 dB attenuation instead of only 12 dB. You really need more attenuation than than to remove drums and vocals and guitar, but 24 dB works fairly well. Sometimes I have ALL the sliders down and just turn one up at wherever the raised octave bass notes sound the clearest.
I've transcribed seven songs for bass lines this way and it really works very well.
The software is cheap, $50 list price, but some internet sites have it for $40. I bought it for $50 and it's the BEST thing I've done in order to transcribe.
Check it out. Let me know if you want more info.
Ciao,
Mauro