i guess if you think about it, there is no such thing as a subwoofer, a sub is just a big speaker powered by an amp to produce pure bass, so you can just bu y a good pair of 6 by 9's and add an amp, but if you want a True sub, they dont make a 6 by 9 sub, its not a "real" thing
Do u really wanna learn something about audio and low frequencies behavior in a car, man... Speakers 6x9 can reproduces sub-low-frequency, with a correct project and power amplification, course. Maybe it don't play so loud as a bass-speaker enclousure or a real subwoofer speaker, but u will ear (or fell) the sub frequencies as well as u ear in a home theater system, for example. In fact, u will be more definition and low effect nuances (in special for songs w exiges clarity reproduccion, like acoustic recordings).