Some of us bought the
https://www.sonicelectronix.com/item...-VPAS12ST.html
I like it, if you're just looking for balanced bass to take the load off your midrange drivers, it's perfect. It can get pretty loud, but I never turn it up much to really know. I just know it can get louder than I care for.
I would HIGHLY suggest replacing your stock speakers first though. The stock stereo is trash. You may actually be happy with the amount of bass without a subwoofer.