the stock headunit puts out about 10w per channel to the speakers and the sub amp is between 20 and 30w if i remember right. might be higher than that, but not by much.
if you're wanting a new sub then you'll need to get a new amp as well. the stock one isn't designed to do more than what it has. you can use the box, but wouldn't only recommend that with a lower power aftermarket setup and would be a good idea to put some dynamat on/in it.
2010 Volvo C70
2011 Harley FLHTK
Midwest Crew #22