When buying a sub and an amp for a car stereo there is a number of things that should be looked at before you even consider purchasing them.

Subs and amps  have various ohm loads and voice coil configurations that they are capable of producing and handling( bare with me). Depending on what amp and sub(s) you have will change your options on the ohm load through power output (amp) and what ohm load to use (subs and amp).

For example subs with a dual voice coil allow for a various degree of ohm loads, a sub that has 2 voice coils set at 2ohms can be wired to handle a 1ohm load or a 4 hm load from an amp. So if the amp is only 2ohm stable it SHOULD not be wired down to 1ohm as it will run the amp to hard and most likely light on fire and smoke you out of your car (cheap amps do this all the time) and if you run the amp at 4ohms you will not be getting as much power as you would from running 2ohms, so a 2 voice coil 2ohm sub should NOT be purchased as it is a poor match.

 A good website that allows the average joe to see how they can wire thier amp is provided by Rockford Fosgate and can be found by searching "wiring wizard" on google, it is the 1st option that is given. check it out for even more options with different subs. Your amp will display what ohm loads it is able to run at, the higher quality amps will be able to run at 1ohms more often than cheap ones will, as cheaper ones will stop at producing 2ohms (usually)

Wire is another thing to look at when purchasing an amp. If the amp is huge and gives out a lot of power you will need larger wires ran for power and ground Ground and power should always be the same size, or the ground larger as that is what completes the circut and you should not bottle neck it by having a large power wire and a small ground. Same applies for the power if you amp needs a lot of current to run and you use a small wire, you are bottle necking it and it is not producing the power that it can.

Thanks for reading!