If your looking to put a sub type speaker behind the HU, you might want to look at a Focal 5WS sub. Its a 5 1/4 sub that really does a nice job. I had one crammed in the dash of my 96 civic and it really sounded great (I would say it was playing as good as an 8'' sub would play) I basicly just built a box as big as i could for it and let'er rip. Not sure on the price nowadays, I paid about 200 for it over 3 years ago.
Hard to think that a little 5 1/4 sub would play that good, but it really does. I will be throwing one in my Alero this summer since there is far more room behind the ac/hu/storage bin area then my civic had.
Here is the link to the
Focal 5WS