You may want to meet up with Dave to have a listen. He may be able to help you figure out what it is that you find lacking and what you may want to improve.
A smaller sub box for your current driver might help. Add stuffing it there isn't any now. I wouldn't both going to 8" drivers. The quality of the output does not depend on the size. A Lambda PB and a new amp would the ultimate upgrade, IMO.
