ive built a custom box which has got a 12" 800W sub in it. the box isnt very deep but is quite wide, now when i have the boot open and music on, the sub seems to sound as if its bouncing off the bottom of the box and unbalancing the sound. would a port of say 70mm be any good to reduce the pressure build up and give the sub a better running performance? if not then what do people suggest?