Ah, root bass is different from the bass in any pattern. I have decribed how to change permanently the bass in any pattern, composer or custom. You can change the bass in any preset quicker with the sound arranger, which is simpler.

root bass is that sound when you play with the keyboard split but with no auto accompaniment triggered in the left hand. You find it by going into the sound menu and either

1 go to part setting and scroll through parts to the end where it is called r.bass or

2 go to mixer and press other parts/tracks under the display where it will be on the accompaniment page called r.ba

in both cases you can now change the root bass sound and anything else you want.

There is only one root bass since it only sounds when no styles are playing. Start a style and the bass voice in the style automatically takes over. Thus root bass has nothing to do with the styles; if you want to save different root bass sounds just do it with different panel memories.