[ParaRiddleDiddle]

Hm. I play jazz bass, e. though...

Anyway, theory helps bass a LOT. Most especially chord theory. Learning and applying common harmonic progressions will help you IMMENSELY. Key notes: in jazz, and a lot of popular music, the chords move in perfect fourths. This means UP in 4th's; for example, if you see Cm7, you expect a chord beginning on the root note a perfect fourth up. In this case, it's a chord with root F. (The quality is determined by key, melody, and ear, and I'm not counting inversions here...)

This is where the ii-V-I comes from. Notice they move in fourths. If you extend this progression, by moving entirely in fourths, you get a longer progression: ii-V-I-IV-viio-iii-vi. Note, however, that there is one tritone interval, from chord root to chord root. (I'll explain that later).

Ever heard of Autumn Leaves? It's that exact chord pattern, in the key of G major. However, Autumn Leaves is in E minor. How is this possible? The two keys share the same diatonic seventh chords. The most important thing about the progression is that it moves in fourths, not the key it is in. (Note also the progression ends on an Em7 - the tonic).

Anyway, learn chord charts - for walking in 4 and 2, and for general comping; for bass, I'd say learning chord charts is more important than learning to read. Chances are you'll be given chords, not melody. (Learn to sight read, if you are playing upright in a classical context. The only way to learn though, is by practice. And get someone to put you under pressure, and force you to read. Does wonders...)

Scales and modes are useful for a bassist, but unless you are big on bass solos, modes won't be all that neccessary. Also, learning chordal and harmonic theory can help you just as much, if not more, in soloing.