If you're mounting something in the trunk of your Optima, I would agree with sticking to at least a single 10" sub. You can amp it moderately low (save a few bucks there) since you are not looking for thunder-just something to round out the low end. As was mentioned, we have all of JL's subs on display here and you can actually run a box out to your car, and we have a speaker wire reel that will allow the display board to drive it. So effectively, you can test drive the sub before buying it. This is particularly useful in a situation like yours.