pre-amp, no sub. connection

how do you hook a subwoofer into a stereo that does not have a sub. output?
Couldn't you just use a pair of Y-jacks on your preamp's outputs to split the signal?
if your amp has more than a pair of speaker outputs you can use the other outputs and a capacitor,in car audio you can take pos. of 1 chanell and neg. of another channel and run that to the sub,that way you get both signals l/r,however im not sure that this is best,just an opinion