If you want to add subs to what you have, you can run a second pair of pre-amp cables and add them to the front or rear channels of your head unit if it has dual pre-outs to a second amp for the subs. If you only have one set of pre-outs, you can use a set of rca splitters ( Y connectors ) and then run a the second pre-amp cables off of the splitter to amp for the subs. The problem with this is you have no front and rear fade capabilities unless you have dual pre-outs.
Depending on what kind amp you have, some of them have pre-outs that you can use to power other amplifiers.
As far as the Memphis subs, I am not sure how good they sound. The best thing to do is go to a stereo shop and have them demo some for you and you be the judge of how well you like like them.
