I bought the Infinity sub http://www.infinitysystems.com/caraudio/category.aspx?Ser=BSL
with the remote subwoofer volume, plus the head unit has subwoofer volume controls and auto level (when you pull up to a light, the volume level automatically adjusts down, then back up depending on ambient noise). I can get just the right amount of deep bass this way. The sub is screwed to the floor with a piece of thick carpet underneath it, which deadens the trunk space, and all objects in the trunk are confined to prevent rattles.

Oh, yeah...the sub takes up less space than the collapsed space saver spare