I have a 2007 hhr and it has the factory sub in it. Have several questions.
If I put a stronger amp on factory sub will it hit harder. Or would it be best just to replace sub and amp to get harder hitting base. I am interested in fosgate sub, anyone put one in their hhr. If so what model. Anyone know how mush air space is in factory box?

 

Most of what you need is in the sticky threads at the top of this forum. From what I remember: the factory sub is 2ohm, so you could replace it with another 2ohm 8" sub, but that wont get you much. You could switch to a 10", a couple members have done so, but the amp doesnt give it much power, so that could be re-worked. And you need to modify the factory box or make your own to fit in the same space, that can be tricky. It is only .3 cubic feet or so, about 4 inches deep and oddly shaped.
The most bang for your buck is to add an amp and large box in the cargo area, but then you might need to remove it to carry cargo. Id go for dual 12"

 

The factory sub wont handle any more power than the factory amp. Also the factory sub is meant to work in the factory enclosure and most aftermarket subs wont work as well in the factory enclosure. So the best way to upgrade is to add a new amp, new box, new sub.