I have a free air 12" sub pointed through the ski hole and it took a while, but I found a black vinyl rear seat with arm rest.
The arm rest seat is made differently than the regular rear seat. The regular rear seat is a solid block of foam. The arm rest rear seat has less foam and appears to have many strands of fibers glued together.
With the arm rest down, I have a vibration in the arm rest mechanism that I haven't bothered to track down. The bass is sufficient with the arm rest up.