If all you are doing is adding a sub, You can use a Pac LOC wih a Volume knob built in to it. This will help compenate for bass volume adjustments at higher levels. Or any LOC with a combination of Sub level control.
For that matter If you have a SUB Lebel knob available for your amp, you can use that as well.
JL AUDIO XD Line of amps are designed to hook up directly to the fatory system without the use of an LOC. They have a hi-lecveland low-level input swith built in. You can also buy a Remote Level control for them to compensate for subwoofer level control.
Unless you are doing a full system, other more expensive devices may not be necessary. They will work better but not necessary when you are hooking up subwoofer only.
Here is a picture of one with sub level built in.
Here is an LOC
Here is a nother one that is adjustable.
