BTW the rear speakers are 8ohm (its a '98 lumina)
this is not the impendance over the whole spectrum and it depents also on the housing but you will have frequenties that the impendance is much lower and higher
I really just want the lower frequencies to be felt by the driver rather than have loud, annoying bass which will piss neighbors off.
to be felt by the driver means you have to work with PA equipment and a lot of power (extra bateries in your car and an amp and suply that is expensive)
pissing neighbours off is basicly the frequency you lisening to
the lisening room from car speakers (cabin) is normaly 2 a 2.5 meter that means that the lowest frequency to be heard in that room is 132 Hz all tones lower you dont hear in the cabin but only feel a 33 Hz tone you will hear 11 meters or further from the speaker
I dont really know if its worthwhile .. or what cutoff frequ a subwoofer should be used at...
it is worth wille to do it if you want to have a good stereo picture
tones are not direction sensetive under the 400 Hz
personaly if take the subwoofer frequency not higher than 250Hz
voices sound beter on your mid tones withoud that part of it is represented in the subwoofer (this is speaker depending)
Robert-Jan