to add a sub the best bet is to go with a powered sub, the only powered sub i would ever recommend is the infinity basslink. all the other powered subs are not worth the money. an after market head unit will have a much better signal to your speakers and many options to add to your stereo. you can run an amp with the factory unit, all you need is a line out converter, i suggest the pac soem t(PAC SOEM-T (soemt) Premium 2-Channel Adjustable Line Out
) it taps into any speaker and sends a mono signal for a mono amp and it also has a remote turn on wire to tell your amp when to turn on. is an after market unit a must? yes, sound quality and adjustability are better than stock, rather than a 3 channel EQ(bass treble and mids) you can get aftermarket units with 7 band eq's. it all depends on your wallet.