gain on factory amp...
 

is there a way to up the gain on the factory sub amp... I find that the base is not high enough

You could always go this route.

https://www.chevyhhr.net/forums/audio-video-17/08-world-finals-install-20551/

lol, thanks, that is a good way to go if I do say so myself.

But there is no way you can turn the gain up on the factory amp. It is set at the factory to work well with the factory sub.
Even if you could turn it up, it would most likely blow the sub.

Best bet is an aftermarket amp and sub.

You cav do a single ten and a small 600w amp and get much better response than the factory sub