There are many things to think about with your subwoofer, including things already mentioned on this website like the type of enclosure that you can have for the subwoofer.
Another thing to consider is the design variations that come in the styles of front-firing, down-firing and passive radiators.
What do all these mean? Well, don’t worry, we will explain it to you.

Front-firing subwoofers have a speaker that is mounted in the front of the subwoofer so that the sound radiates from the side or front of the subwoofer enclosure.
Down-firing subwoofers use a speaker that is mounted so that it radiates downwards to the floor of the car. For these types of subwoofers, you will find that some enclosures will actually use an additional port to force more air out, which then increases the amount of bass you feel and it does so in a more efficient manner than sealed enclosures. This means you don’t need an amplifier to boost the signal to the subwoofer.

Passive Radiator subwoofers use a passive radiator with a speaker instead of a port. This increases the efficiency of the subwoofer and gives it much greater precision as a result. In terms of what a passive radiator is, it is usually just a speaker that has had the voice-coil removed, or in some cases, is just a flat diaphragm. Systems that use a passive radiator are very similar to ported subwoofer enclosures. If you want the quality of the ported subwoofer enclosure, but you don’t have the space, then a passive radiator subwoofer will work great because it expands the car stereo system’s ability to hit the low frequency sounds, without the extra space needed by ported subwoofer enclosures.

In terms of what will work best and have the best quality for audio and bass in your car, there actually is not too much difference between front and down firing subwoofers. What you should consider is the placement of the subwoofer. If it is an free-air subwoofer, then you will have to think about the speaker accidently being punctured or hit if it is on the rear rack, and in the trunk. Therefore, having a down-firing subwoofer may work best for you to keep it from getting damaged.

There is so much to consider when you are picking out a subwoofer, that knowing what you want before hand in terms of quality and placement will go a long way to ensuring that you don’t get something that does not work exactly as you had hoped.