A high proportion of pre-programmed MIDI files of commercial backing tracks will conform to the (Roland) General MIDI GM/GS format. This convention dictates that the drum and percussion note data will be on MIDI channel 10. The bass instrument note data could appear on any of the other fifteen tracks.
In whatever you are using to play back the MIDI song files, the individual parts should exist in individual tracks and using individual MIDI channels. The majority of DAW software and dedicated hardware MIDI Song File players will have a Mute button per track. Use these to silence all of the instrument parts that you do not require. Save a copy of the song file in this state under a different name from the original commercial file.