Well your already at about the best possible box for lows, more port area might let some more lows out. Tuning it any lower just kills efficiency, and any larger box is going to kill the 45hz plus, which r's already **** at. Without upgrading your amp thats the most lows your going to get out of it. Mine 4 were still too low end monsters in 15.8ft each at 37hz so maybe your box is calced wrong or the car just doesnt have any low cabin gain. Blueberry isnt low but if you want that low in king kong, get 1000RMS, tune to 33 and try 45in of port area.