I would say it depends a lot on what the rest of your system is. I wouldn't even bother getting a sub before upgrading the rest of the system depending on the spec of what's already there, because the sound will be too heavily shifted towards the lower frequencies. If you are already happy with the rest of your system, sound testing is the best bet, the only problem being that you need to do the sound test with a system that is as near as possible to what you already have. One sub may sound great with one system, but not so good with another, the various parts of the system need to complement eachother. The other problem is that the acoustics of the car and the positions of the speakers will also affect sound quality and performance, so what sounds good in a listening room will not necessarily sound good in your car. Position of the sub is irrelevant because very low frequency sound is not affected by directional output in the way that higher frequncies are, but the position of your other speakers will affect how well the sub works with them. The other thing to consider is the frequency range of the existing speakers, if they go quite low anyway you would do best to avoid to much frequency crossover, because it will detract from the clarity. Ideally get an active sub or at least an amp with a specific sub channel which will alleviate too many higher frequency signals being sent to the amp. Make sure you use a good quality subwoofer cable, as you will not get the best out of the sub with crappy cabling. Power source is also important, upgrade the battery and alternator for best performance. Mounting of the sub is also very important, do not mount it directly to the boot without some form of damping, preferably using material that absorbs resonance.
On the other hand, if you are not an audio purist, just go for the loudest kickass, bassiast setup you can find.