the factory sub will be 2 ohm, and they make them very high effiecency, so they don't need as much power.
Replacing the sub will give you less volume, so you'll turn it up. Guess what happens next?
Distortion, from clipping, which will destroy any good sub. and then you'll wonder why it sounds like shit.
Buy an amp, and sub. Install it properly.
Be happy.

__________________
Alpine CDA-9887, TUA-T500HD, CHA-S634, CHA-1204 & KCA-410C
Pioneer TS-A6981, 6 x GM-5100T
Kicker 2 x CVR15,
MBQ RCE-216
PG MX2
Soundgate LanStar1
PAC SWI-ALP