The setup of an instrument is possibly the most critical factor to the performance and development of the musician. To touch on some phrases you're probably heard: "I played on a $100,000 bass at a convention and it didn't sound good", and "you can get a bass mail-order for $800 - why pay more?"
The response to both these issues is 'setup' - also known as adjustment, a labor-intensive, expert procedure and body of knowledge. An inexpensive bass may never sound as grand as a fine instrument, but you don't have to sacrifice playability. A fine instrument may have a stellar pedigree, gorgeous wood, and everything going for it, but if somewhere along the way an owner cheesed out on some setup work, the magnificent bass won't sound to its potential. There is no substitute for a good setup with quality components. And - it's not a plug for our repair department to state that dollar for dollar, it is actually less expensive to buy a new instrument that has been set up by a top-notch bass luthier with a quality bridge, professional strings, and other appropriate fittings than to order a bass that already has labor invested in an inferior setup or a cheap quality bridge that will have to be replaced.
Like the drive train in a car, the setup on your bass involves the 'moving parts' (more accurately, the replaceable parts) - the bridge, sound post, tailpiece and tailwire, endpin, fingerboard, nut, and saddle. The two largest components of this drive train are the fingerboard and bridge, as if either of those is misshaped, the bass simply will not play properly. In fact, if the fingerboard is not properly shaped, it makes it virtually impossible to play that instrument in tune (as a simple matter of physics - variations in the fingerboard surface directly affect the vibrating length of the string). Nor will the bass sound to its full potential if either the bridge or fingerboard is made from soft, inferior wood. A bridge requires more than just the top edge being cut to a correct curve - bridges, like the top and back plates of the bass, are graduated (cut thinner in specific places) and shaped to complement the bass, the strings of choice, and the player's style. A bridge for solo playing, for example, is shaped quite differently than one for orchestral playing, and the mass and shape of the bridge have a big effect on the tone and resonance of the bass. As such, a bass that is inherently bright or dark can be 'normalized' to a degree with the help of a properly graduated and shaped bridge.
The quality of the spruce sound post does make a difference, and the fit and position of the sound post are critical, as most players know. Basses change shape significantly in response to temperature and humidity - often expanding or contracting by more than 1/4 inch! Also, even when perfectly fitted and the bass is under tension, some sound posts have a mysterious way of migrating around, and do have to be periodically put back into their happy place.
The tailpiece should be well matched to the bass. The (very) general rule is that if the top of the bass is thick or rigid, the bass will do well with a lighter weight tailpiece, and vice versa - those with a thin or very flexible top sound better with a more massive tailpiece. Like diets, there is not a "one fits all", and though from time to time there is a big buzz about light plastic tailpieces, low-mass wire tailpieces, or harp-shaped 'compensated' ones, it's good to seek the advice of your luthier on whether any specific appliance will benefit your bass. All of those types can be helpful in the right circumstances, and with some basses, none are really the best choice.
A tailwire which is the proper length to tune your afterlength (the short length of the strings between your bridge and the tailpiece) is one setup feature that shouldn't be underestimated. When the afterlength is a consonant pitch relative to the open strings, wolf tones are reduced "naturally" (without having to resort to a wolf eliminator), and basses most always have more low frequencies and a more open sound.
The height of the saddle is a very important factor in the perceived string tension of an instrument, but as it's a little more labor-intensive than many other adjustments, make sure your bass has already been adjusted to its optimum and you have played it in significantly before asking to have a new saddle made. The luthier may find that your bass is either too tight or too loose for other reasons apart from the height of the saddle.
Strings are possibly the most discussed, most debated factor in the bass world. Every string on the market has players who love them, and players who swear they are garbage and they just wasted $200 - why? A great deal has to do with matching the strings to the bass, firstly, and adjusting the bass to work optimally with a given string. Some strings become hugely popular because they are almost a panacea - improving almost every instrument - but then, there are those basses which are just not compatible with that string. In almost every case, switching brands of strings requires a soundpost adjustment, and often a bridge adjustment, when the strings are installed and again after the break-in period. When you choose a new brand of strings and find they are perfect, it indicates your sound post was in a great spot to work with those strings, or in other words, it was in a bad spot to work with your old strings. There are strings that offer wonderful nuance - and require more of the player than strings that are 'forgiving' and cover technique issues. When deciding on strings, a luthier's advice as to what will work with your bass is valuable, as is your teacher's advice about the technique that works best with those strings. Just about every healthy, thoughtfully chosen upgrade to your bass comes with the challenge of meeting it with improving your technique. If you have a perfectly cambered fingerboard, for example, but the strings are still buzzing when you play, check with your teacher to see if your bow arm or your pizz technique is driving the strings into the fingerboard instead of producing a round, open oscillation of the strings. The best outcome is when you and your bass are the best they can be.
This leads to a mention of the most important factor in instrument setup - awareness! Feel free to ask a lot of questions when your instrument is being worked on. A reputable bass luthier can explain how things work best and why. When your bass is in optimal adjustment, take special notice of the bridge and sound post position, wolf tone, the afterlength pitch, and so on. If the bass suddenly changes and the G or E string starts hitting the fingerboard, you will know if the bridge has been knocked out of place. If all the strings are suddenly much lower, you will know that a seam has come open near the sound post. Once you know how the bridge looks when it's centered and at right angles to the top of the bass, you become an invaluable ambassador to string players everywhere - especially if you are in school. Replacing a violin bridge that has warped due to being allowed to lean over costs $50 or less, whereas replacing a warped bass bridge costs around $250 - which means schools almost invariably choose to fix five violins in favor of repairing one bass. You can look down your section and help other players learn to keep their bridges straight, saving your school a ton of repair money, and importantly, insist the violinists keep their bridges straight so the school can afford to get your bass new strings!
As time goes on, we will get more into detail about the physics of each of the components of the bass. I hope this overview serves to give you an idea of the setup concept. Happy practicing!