Further to DeQ's comment on the rear parcel shelf, the previous owner of my car had cut huge (10") holes in the rear parcel shelf and the speakers were not sealed to the shelf leaving a gap for air to pass through (and probably reducing bass response from the speakers). I have since completely replaced the shelf.
Speaking of speaker holes. My e3's PO had speaker holes cut into the sheet metal below the front of the rear seats. I've noticed over the years that this is popular area to butcher just to add speakers. I suspect these could be a culprit in my car. I ditched the speakers and went back to stock parcel shelf units sans holes in the shelf. I'm planning to seal holes below the seat and install sound deadening. Currently only covered with carpet.