I recently brought a streight through exhaust (cat back) with a peco Jap 4" backbox. Fitted it all and its realy loud (more than expected) and quite hi pitched. I have now kept the backbox on and refitted the origional centre section, the sound is much deeper now but quiet. How can i make my stright through deeper in note but not as loud?? I was thinking of a universal centre box (stainless) but what size (dimentions) would i be looking for. any one help??