The orchestra has several instruments which can used to form the bass:
Cello: This should be your standard bass instrument.
Double Bass: This instrument supports the cello. It usually plays the same note as the cello but one octave lower. I would recommend to stick to this principle at most times. Other than the cello, it can not play fast passages well. Keep that in mind.
Basoon: This is the standard bass instrument for the woodwinds. It goes very well unisono with the cello.
French Horn: The French Horn has a wide range and can also be used to form the bass. As for timbre, it can be seen as a bridge between the woodwinds and brass.
Tuba: This is the standard bass instrument for the brass.
Trombone: If a powerful and/or threatening bass is desired, this instrument will do the trick.
As you can see, there are a lot of possible instruments for forming the bass in an orchestral piece. But be sure not too use too many bass instruments at the same time, as it can sound very dull and boomy quickly (especially if you add reverb to it). Find a balance between the high and low notes. The more high notes there are, the more bass you can use.
Chords and close harmonies in the bass are a “no go”. Playing a full chord with cellos ore any other combination of bass instruments can sound dull and boomy quickly (especially with reverb). Use single notes to form the bass, that is, the bass instruments in unisono, plus/minus a full octave.
- Bass line
Having the bass always play the keynote of a chord is possible. It often sounds better though, if you construct a series of neighboring notes for your bass as the chords change. For example, if you go through the chords Am – Em – Dm – Am in that order, the bass A – E – D – A will always work. But here you could also use the neighboring notes A – G – F – E as all of these notes are in the corresponding chords. You can create nice effects with such ascending or descending bass lines.