I've been trying to think of the best way to describe this without going into a lengthy post.
Trussrod: For adjusting the small amount of forward bow (relief) that all basses must have for the strings to vibrate properly at the middle fret areas.
Saddles: For adjusting the actual string height.
Neck tilt adjustment or shim (bolt-on necks): For changing the angle at the neck pocket, which can bring the upper frets closer to the strings, resulting in a more even action up and down the board.
As Metal Mitch stated, don't think of the truss adjustment as an action setting tool. Yes, the action will change when you adjust this, but all you're trying to do here is get the right neck relief. It doesn't matter if your saddle action is high or low, and you don't need to lower the strings to get them buzzing to begin your adjustments. The relief will not change because of saddle height adjustments.
You'll eventually run across a bass that has an action that you just cannot fix by adjusting the truss and saddles alone. If you're thinking that you can even out the action at the higher frets by tightening the truss, this does not always hold true. Alot of that high action has to do with the fretboard's angle to the strings. You might end up with a properly adjusted neck relief, saddles as low as they can go without fret buzz at the lower frets, and still end up with an overly high action at the upper frets. A neck shim (if it's bolt-on) is in order here to cure this. Don't think that tightening the truss more is going to fix this. It won't. All you'll be doing is removing the PROPER RELIEF from the neck, causing more problems at the lower frets. Even if you somehow stumble across an action that feels good, notes on the fretboard just won't sound right. Some will sustain, others will not, and intonation will be even harder to set.
Ok, let's say you finally arrive at a good setup action. In the future, you'll only have to make a minor truss adjustment to keep that action, unless you change string type or gauges. You won't need to mess with the saddles. They are already set. If you found your action a bit too high one day, maybe due to temperature changes, all you would need to do is tighten the truss just a bit, checking the relief as you did in the beginning. The reverse holds true if the action went low.
Remember that anytime you change tune or tension on the strings, you're also changing tension on the neck. So when you check relief, always make sure the strings are tune to the pitch you intend to play at.
Hope this helps..
Mag...
