I'm pretty sure when I pulled the factory sub it was already running at 2ohms, so unless you but a dual two ohm and ran it at 1ohm you wouldn't get any more power out of the factory amp and it probably wouldn't sound good. There are a lot of amps out ther now that will let plug high level inputs (speaker input) directly into the amp so you don't have to replace the HU.
So I think as a minimum you need a new 8" subwoofer and an amp with high level input and it will give you quite a bit more bass then your getting now.
Optimally though, a new box, 10" sub, HU, amps, and new components are really necessary to get good quality sound.
Hope that helps.
1999 SS M6 #1935 (Dad's)- 387hp/385tq -12.25 @ 113
1999 Mustang GT (Mom's)- 325hp/369tq - 12.98 @ 105.3
1995 Z28 A4 (Da Boy's)- Custom RAM, shorties, cut out
2014 Chevy Cruze LT/RS (Da Girl's) - Stock...duh
2002 Tahoe LS- Off Road Wheels/Tires, Flowmaster (duals)
2011 Coachmen Mirada (Big *** rolling turd)- sleeps 7
1991 CBR1000F (commuter bike)- 43MPG, up yours Prius!