I just tried a 150-watt 10" Bazooka in the trunk, and it's effect was in my opinion significant enough (not a "wow" effect), but boomy. Despite different crossover frequencies, nothing changed. I want a deep, clean addition to an already quite descent system, and the bazooka ain't it, unless perhaps it was mounted behind the drivers seat, which I've seen done before to great effect. The place I went to now suggests a Boston Acoustics 10' enclosure with a seperate cleaner 150-watt amp (the name escapes me). The box is about a foot square, and could be pointed toward the trunk opening.
What do you think?