If you want MIDI drums and bass, you should select an appropriate style. If you choose a bass RealTrack and a drum RealDrum track, then the style does not matter. In either case, just mute the instruments you do not want to hear. Note that, without entering the part markers for a particular song, you won't hear drum fills in the right places; there is no way that I know to control that in real time.

One way to mute a track is to click on the instrument button, then right-click and select Mute.

Tip: the Mute setting does not save with the song, so one way is to set the volume of the track to zero before saving.

There are other ways to do these actions as well, but first see if this gives you what you want.

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