Ive always had bad luck with factory amps.
Things are just never the same. Considering they use high level inputs for signal. The levels output by the new HU are well over the stock, therefore more distortion is a real issue.
I wouldnt waste time with subs in the rear deck of the car.
If you want to go with subs Tang Band makes two different 6x9 subwoofers. Please note someone got them and they wouldnt directly fit into our plastic adapters- so some slight modification necessary.
http://www.parts-express.com/pe/show...number=264-864
http://www.parts-express.com/pdf/264-864s.pdf
Heres a good amp then that will power thos rears correctly, now to find some fronts:
http://americanbassusa.com/front/sho...aspx?fileid=81
I could do these for a decent price:
http://www.massiveaudio.com/store/ck...-pr-16176.html
Outside of that a nice set budget to work within. Just beware, if you state $600 Im going to look for the best ones AT 600$
You could get both subs, an amp, speakers and wiring kit for around $500 or so. Just ball parking it.