I have seen many questions about how to wire up subs, the difference between parallel, series, and bridged. I have posted some diagrams with explanations.
The first is the parallel/bridged configuration. If you are looking for the most boom from the output power you have, this is the way to go. However, you must be very careful with your impedance. When you wire up subs in parallel, you will be cutting the rated impedance in half if you wire two subs of the same impedance in this way. For example: If you have two 2 ohm subs, wiring them in parallel will cause the overall impedance of the circuit to be 1 ohm.
This is the bridged/series configuration. This is used if you have an older amp that can't handle low impedance subs. Say you purchased a new set of subs that are 1 ohm each, but your amp can only go as low as 2 ohms. By wiring subs in series, it's like adding impedance with each sub you wire into the line. So, to make up for the limitations of your amp, you can wire the two 1 ohm subs in series to make the circuit 2 ohms. This can also work the other way around. Say you have a new amp, but your subs were not designed for the increased power output. You can wire them up in series to prevent them from being fried. Also, if you are concerned about distortion and sounds quality, this is the configuration you should use. The higher the impedence, the more the amp has to drive to go into clip.
thanks to Bob!