Why the hate for 6x9's on here? in fact Sony ones are very good quality wise, wonder what people think of 7x10's? personally OP i'd invest in some very good 13's and keep them in the standard pods with a bit of Dynomat around the fitting to absorb resonance it will make the quality much much better, decent ones that hit a low Hz range will produce bass, 30Hz is widely available.
CAS sell a powered 10" Sub for under £100 and the sound quality is very good, if boot space is an issue for you go for the powered Alpine Under Seat Sub, it's perfectly fine for everyday use, if you've never had a monster Sub in the boot I can tell you it will grate after a while on day to day use and the Radio becomes horrible with all the static rasping through it, the build you make affects the quality massively such as what L33thtr33 above has said, use decent materials over placing a sub over the carpet and will feel more pro and give you much better bass.