I understand theory so you don't have to post individual notes. I get that you can play an octave or using fifths but how do I know what note to use in a specific case.
....without posting notes all we'd be posting is theory. Anyway, you're probably talking about substitutions. In this case, take whatever note you play in the bass and try these instead:
1. Playing 1/2 step up or down
2. Playing a whole step up or down
3. Playing minor 3rd up or down
4. Playing major 3rd up or down
5. Playing a 4th up or down
6. Playing a b5th up or down
7. Playing a 5th up or down
8. Playing a 6th up or down
Try all of these with your chords then do the opposite, moving the chord your playing at those intervals while keeping the bass the same. You can also try inverting the chords you're playing. I know you're asking about bass notes, but changing the chord sometimes make the bass sound different too.
Hopefully, this is what you're asking about. Also, note that you can add notes to the bass (as you were saying) like a 3rd, 5th, etc., all while moving the note around like I showed ablove.
