Up front bass is the psycoacoustic (that's a big word, huh) phenomenon (another big one) that makes us think that our rear mounted subwoofers are actually in front of us. Hence the term "up front" bass. Basically it's a way to trick your brain into thinking your subs are in the front of your car audio system. This is very desirable for sound quality listening so that all your music appears to come from the same place (in front of you). You don't want the high hat to come from the dash and your kick drum to come from the rear.
There are several factors that need to be achieved in order for this trick to work. First, we need a car audio subwoofer system that is capable of producing low frequency bass. This is not a problem for most systems due to cabin gain. Second, we need to control the frequencies that this subwoofer system produces. Specifically we need to cut off the high frequencies from the subwoofer system. The human ear isn't able to detect the location of low frequencies nearly as well as it can high frequencies. By limiting the subwoofer system to play only the lowest frequencies (those we cannot locate by ear) we have the first part of the equation, low bass only. This is achieved by using a subwoofer crossover frequency in the 40-60Hz range combined with a very steep slope of 18-24dB per octave.
Now that we have our subwoofer system producing only the lowest frequencies we need something to fill in from the those frequencies on up. This can be in the form of a door mounted midbass speakers or smaller subwoofers mounted up front (usually in a small enclosure located in front of the front seats). If your front speakers are capable of reproducing frequencies down to 50Hz with authority (and really very few do) then you won't need separate midbass speakers in the door or custom enclosures.
You can also achieve bass up front by having your subwoofer system up front. If you have a vehicle with a rear/mid mounted engine with the trunk in the front then that's where you can place your subwoofers. Clearly this is the easiest way to go but very few vehicles have this option. The other option for front mounted subwoofers is to place woofers with small enclosure requirements (usually an 8" pair) in custom enclosures mounted just ahead of the front seats (in the area between the seat front and the back of your legs). If you go this route be sure to secure the subwoofers so they don't want to slide forward. If more than one driver uses the vehicle, and they adjust the seat before driving, have the enclosure mounted in front of the seat when it is in the forward position.
One of the quickest way to ruin the illusion of up front bass is to have the rear of your vehicle rattling. It doesn't take your brain long to figure out that the subwoofer is in the same place as the rattling body panels. Take care of these noisy panels by using a combination of sound deadening sheets (Dynamat or similar), expanding foams and other sound deadening materials. Then using a test tone CD, sweep through the bass range and see if certain frequencies cause other panels to vibrate. Sound deaden these areas and repeat until your rattles are taken care of. Then sit back and enjoy your new car audio system. Just make sure you leave enough juice to start the car.
You should also check out Advanced Enclosure Design and Fabrication. It covers designing enclosures using free computer software, maximizing the output of a subwoofer system, building the box like a pro and testing the output using inexpensive equipment. Click here.