Requirements:

1.) The Amazing Slow Downer ($49.95), then 
2.) Your Digital Audio Workstation (D.A.W.) or Audacity (free)

The Amazing Slow Downer  will be used to create a mono .wav file of the slowed bass and a separate file of the slowed drums. 

Your Digital Audio Workstation (D.A.W.) or Audacity will be used to blend the bass and drums to a stereo or mono mix.

Settings:  After purchasing and installing The Amazing Slow Downer from http://www.ronimusic.com/ choose “Options” then “Preferences” with the following choices:

Sample Rate: 44100 Hz
Play Buffer Size:  (start at) 4096 Samples (you may have to adjust this later based on the processing power of your computer)
Read Buffer Size: Big
Slow Down Quality: 3  (even though you are not “slowing down” the track, use this setting)
Checkbox for “Pitch Change affects speed”:  leave unchecked

(Then click “Okay”).

Transpose the bass from a DrumAndBassTracks.com file. 

1. Use the “Mix” slider to pan the bass all the way to the right.  This way, only the bass will be processed as it is mixed to the right side in the stereo field.
2. Left click “File” – then “Open File” and choose the backing track you would like to speed up or slow down.
3. Use the “Speed” slider to change the key in percentages.
4. Use the “Play” button to test and to make sure you are only hearing the bass.  Click Stop or the "Save Song As" choice will not work.
5. Go to “File” – choose “Save Song As” - and choose “Wave File”
6.  Make sure to set the following parameters:  Repetitions – “1”; Audio Type – choose “Processed mono”.  Then click “Okay”.
7. The “Save Song As Wave File” box will then open.  You may wish to rename the file to the new key you are transposing it to i.e. “Any Song – mono Bass in (key - TEMPO CHANGE)”.  
8. Then click “Save” and wait for it to process.
10. You have now created a mono bass file in your desired key. 
11. Use the “Mix” slider to pan the drums all the way to the right (if using a stereo drum file, leave slider in center).  This way, only the drums will be processed as it is mixed to the right side in the stereo field.  Follow steps 2 - 8.

The following can be done use your D.A.W. or Audacity (instructions written for Audacity but easily translate using your D.A.W.)

To create a Left/Right Drum and Bass file at your new tempo using Audacity:

1. Open Audacity
2. Go to “File” and “Import” the two files you just created.
4. Pan both tracks to your preference.
7. Export.  Rename to save confusion.

Using The Amazing Slow Downer for this whole process will not return the best results and will give several drop-outs in the drums and bass of a Drums-Left / Bass-Right file.