If you are investing in a sub, DO NOT run it from your head unit. Please, at least get a single channel amp for it. But really, if you are going to get a sub, upgrade your speaks, or at least have them running off an amp too.
I reworked the audio in my car a little over a year ago.
Front two speakers (5.25") run off the head unit
Rear speakers (6.5") are connected to a 4-channel amp
Subwoofer (10") connected to the amp (bridged)
Amp is 720w
180w going to each rear speaker
360w going to the sub (because its a bridged connection)
If you only get a sub, you will be missing the entire midrange of majority of songs. If you only listen to bass music, i mean ONLY, then just get a sub or some shit.
An amp is essential imo. The more channels it has, the pricier it's gonna be. If you don't upgrade speaks, you could set something up for ~$300-400 but don't go cheap on car audio. It will degrade down the road. pun intended.