David,
I don’t think you’ve got much scope for improvement. With the roof down it sits on the speakers so even up grading the rears you’ll still muffle the sound. I do remember seeing a series 3/4 where the owner had installed a bass speaker in the bulkhead between the seats. This might work if you upgraded the fronts.
Not a easy fix I spent some time looking at what could be done when I had my S4 good luck
