you could use them as a subwoofer,, although it would be kinda pointless..
reason being that even if you hook and amp up to the speakers and run a low pass filter etc... the bass response of the speaker will still only be as good as running it full range, the speakers characteristics will not change you are just eliminating certain frequencies.. 6x9 are not the best at anything they are just a good all rounder. 6x9's mounted in the parcel shelf and correctly phased can produce a nice amount of bass,, ok not enough to rattle your teeth out but good enough for general listening. my recommendation would be to run them direct off a head unit in the parcel shelf, or amplified with a full range signal for flexibility.
if however they are already in the shelf then u really need to get a dedicated subwoofer.
Edited by THedooBZ, 20 May 2009 - 12:31 PM.