whitelightning05 has the right idea. But that way though easier to understand and hook up(i used to install system's for a living), is the old school way of doing thing's. These car manufacture's have improved factory sound but is still limited. And it all start's at the head unit. It made to protect your factory speaker's. So when you hook up an amp to the rear speaker's it's controlled through ur head unit and is highly influenced by the bass and treble controls. Your best bet is to purchase a sound processor. I bought the MTX re-Q and it's a miracle worker. Right now Crutchfield.com has it cheaper than when I bought it from there but it's the same one. Even though it's inexpensive it's really good. I'm used to have the big 12's and 15's but since I bought my 07 g6 I didn't want a huge box limiting my trunk space. So I bought the powered Visonic 12" sub that already come's in a box with an amp from walmart(yes walmart). It's only rated at 250 watts, but that's RMS power which is true power. I only have one cause my wife dosen't like the big bang but it's still a good choice especially hooked up to that sound processor and the woofer is only 50 buck's already installed in a nice box and the amp built into the box as well. You can very easily install two. You still need the wire coming from the batt. for each one but it's a good inexpensive to go.