MIDI - yeah, you kind of have it right. It’s a MIDI type 0 file, and channel 10. But a BB file is not really a MIDI type 0 file, since the information on it is all in one channel. The BB works like a sampler. You put samples into it, and trigger them with playback from the notes that the sample is assigned to. Unlike a standard keyboard which may have up to 88 keys, the BB gives you access to the full 128 locations in MIDI, normally called C minus 2 to G 8, or MIDI 0 through 127.
So, when we have a bass assigned to midi 0 through 31, C-2 through G0, the bass is not playing back C-2 through G0. Jeez, that would be awfully low. No, the bass plays back, usually C1 through G3, the equivalent of what is on an electric bass. It does that, because those samples have been recorded and assigned to those notes. The BB only play back the samples that it is given, it does not make octave transpositions on its own, or take a sample and be able to move it up or down from its original recorded location. In that sense, when using a 0 through 31 kit, the BB is made to work on something or the order of a split keyboard. Bass is assigned 0 through 31, drums to 34 through 71, for example, and maybe a keyboard at 72 through 127. But, only the drums are in their normal locations. The sounds being plaedy back at the higher and lower end of the midi range are sounds that have been assigned to those notes, and are typically octaves higher and lower than what the assigned midi note would play, were it just playing a keyboard.
All that is to explain why when creating a “with bass” song for the BB you have to do two things after you have your bass and drums sounds correct in the original creating software. 1) You have to move the bass up or down the required number of octaves to match the BB kit that you intend to use; and 2) you need to merge the bass and drum tracks into one track. Now, this track does not need to be on midi channel 10. Midi channel 10 is traditionally assigned to drum parts, but as far as the BB is concerned, when it gets a track all on one channel, it cares not what the channel is.
I’d recommend you try this to merge your tracks. First, create a new blank track. Then, copy your bass track, paste it into the blank track, and move all those notes down 3 octaves. Then, copy all the notes in your drum track. Paste those into the new track.
If possible, export just that track. If your software cannot do that. Save your work with a unique identifying name. Then delete the two tracks that contain the original drum and bass parts. Export what it left (It should be just your merged track) as a midi file.
Load that track into a BB sng. Try it out with a 0-31 kit.